timestamp decimal cutoff not fully applied in backend-generated label files

Backend-generated label files may still contain timestamps with more than 3 decimals.

Example file extract:

      "t": [                                                                                                                                                                                                        
        9.389,                                                                                                                                                                                                      
        9.468,                                                                                                                                                                                                      
        9.546,                                                                                                                                                                                                      
        9.626,                                                                                                                                                                                                      
        9.709,                                                                                                                                                                                                      
        9.779,                                                                                                                                                                                                      
        9.831,                                                                                                                                                                                                      
        9.907,                                                                                                                                                                                                      
        9.988,                                                                                                                                                                                                      
        10.065,                                                                                                                                                                                                     
        10.096,                                                                                                                                                                                                     
        10.147,                                                                                                                                                                                                     
        10.229000000000001,                                                                                                                                                                                         
        10.308,                                                                                                                                                                                                     
        10.388,                                                                                                                                                                                                     
        10.473,                                                                                                                                                                                                     
        10.553,                                                                                                                                                                                                     
        10.629,                                                                                                                                                                                                     
        10.709,                                                                                                                                                                                                     
        10.790000000000001,                                                                                                                                                                                         
        10.868,                                                                                                                                                                                                     
        10.948,                                                                                                                                                                                                     
        11.028,                                                                                                                                                                                                     
        11.106,                                                                                                                                                                                                     
        11.19,                                                                                                                                                                                                      
        11.267,                                                                                                                                                                                                     
        11.34,                                                                                                                                                                                                      
        11.389000000000001,                                                                                                                                                                                         
        11.469,                                                                                                                                                                                                     
        11.548,                                                                                                                                                                                                     
        11.628,                                                                                                                                                                                                     
        11.700000000000001,                                                                                                                                                                                         
        11.748000000000001,                                                                                                                                                                                         
        11.827,                                                                                                                                                                                                     
        11.908,

This issue actually regards the Python taggingbackends module.