Added Django 3.1 support

[FinalAngel]

• Added support for Django 3.1
• Dropped support for Python 2.7 and Python 3.4
• Dropped support for Django < 2.2

[codecov-commenter]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #70   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          176       162   -14     
  Branches         9         7    -2     
=========================================
- Hits           176       162   -14     

Impacted Files	Coverage Δ
djangocms_snippet/admin.py	100.00% <ø> (ø)
djangocms_snippet/migrations/0001_initial.py	100.00% <ø> (ø)
djangocms_snippet/migrations/0002_snippet_slug.py	100.00% <ø> (ø)
...cms_snippet/migrations/0003_auto_data_fill_slug.py	100.00% <ø> (ø)
..._snippet/migrations/0004_auto_alter_slug_unique.py	100.00% <ø> (ø)
...rations/0005_set_related_name_for_cmsplugin_ptr.py	100.00% <ø> (ø)
...ocms_snippet/migrations/0006_auto_20160831_0729.py	100.00% <ø> (ø)
...et/migrations/0007_auto_alter_template_helptext.py	100.00% <ø> (ø)
...ngocms_snippet/migrations/0008_auto_change_name.py	100.00% <ø> (ø)
djangocms_snippet/apps.py	100.00% <100.00%> (ø)
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9db4a48...8f5cf00. Read the comment docs.