remove f-strings for py2 compatibility

13 jobs for master in 1 minute and 19 seconds (queued for 3 seconds)
Name Stage Failure
failed
python-2.7-django-1.11-test Test
          ^
SyntaxError: invalid syntax


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Ran 1 test in 0.000s

FAILED (errors=1)
ERROR: Job failed: command terminated with exit code 1