Add Django 3.1 support

[FinalAngel]

• Added support for Django 3.0
• Added further tests to raise coverage
• Fixed smaller issues found during testing

[codecov-commenter]

Codecov Report

Merging #24 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #24   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          141       132    -9     
  Branches        12         8    -4     
=========================================
- Hits           141       132    -9     

Impacted Files	Coverage Δ
djangocms_audio/migrations/0001_initial.py	100.00% <ø> (ø)
...ngocms_audio/migrations/0002_auto_20160825_1821.py	100.00% <ø> (ø)
djangocms_audio/cms_plugins.py	100.00% <100.00%> (ø)
djangocms_audio/models.py	100.00% <100.00%> (ø)

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