avoid using tkinter with matplotlib for sphinx autodoc

ippisite/docs/source/conf.py

@@ -18,6 +18,10 @@
 import os
 import sys
 
+import matplotlib
+# avoid using tkinter with matplotlib
+matplotlib.use('agg')
+
 sys.path.insert(0, os.path.abspath('../..'))
 import django
 os.environ['DJANGO_SETTINGS_MODULE'] = 'ippisite.settings'