Fixed flake8 errors.
Merge pull request #1225 from normanjaeckel/Flake8Errors

Fixed flake8 errors.
Fixed #551: Used unicode font for circle in ballot pdf. Removed pillow.

That fixed also #1140 (Server error when trying to view ballot paper pdf)
Merge pull request #1215 from emanuelschuetze/fix-551

Removed pillow. Used unicode circle font. (Fixed #551 and #1140)