Make the app compatible with Django 3

[Guts]

On a le temps avant que la branche 3 ne soit LTS mais autant anticiper :

[codecov]

Codecov Report

Merging #130 (bf7c816) into master (87f7a78) will increase coverage by 17.03%.
Report is 1 commits behind head on master.
The diff coverage is 0.00%.

❗ Current head bf7c816 differs from pull request most recent head 9b94054. Consider uploading reports for the commit 9b94054 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #130       +/-   ##
===========================================
+ Coverage   29.69%   46.72%   +17.03%     
===========================================
  Files          82       45       -37     
  Lines        2489     1466     -1023     
  Branches      194      122       -72     
===========================================
- Hits          739      685       -54     
+ Misses       1729      760      -969     
  Partials       21       21               

Files	Coverage Δ
elgeopaso/cms/views.py	0.00% <0.00%> (ø)

... and 74 files with indirect coverage changes