Don't force python3

Using "python" instead of "python3" makes it possible to use the
makefile on systems lacking Python 3.
Use aware datetime objects in demo DB

Fix this warning when running "make demodb":

  RuntimeWarning: DateTimeField Vote.pub_date received a naive datetime
  while time zone support is active.

cf. https://docs.djangoproject.com/en/dev/topics/i18n/timezones/
Add closed votes to demodb

This makes it easier to test closed votes
Display vote ratio with at most 2 decimal digits
Improve ergonomy of the voting page
Remove useless <ul> tag in vote list page
Modify the list of fake accounts for dev

Sync with django-authgroupex
Map group membership to django user categories
Allow superusers to close votes from the main UI
Add a link to administration for superusers
Introduce restricted votes
Convert the voting view to a class-based view
Improve French spelling of the voting button
Improve ergonomy of admin interface