Addon code alignments

[FinalAngel]

• Added isort and adapted imports
• Adapted code base to align with other supported addons

General

• Update changelog
  Adapted code base to align with other supported addons
• Add contributors text to README file
  We're grateful to all contributors who have helped create and maintain this package. Contributors are listed at [link to https://github.com/divio/djangocms-video/graphs/contributors].
• Check that python, django, django-cms and other versions are listed correctly (add badge)
• Make sure Django is not pinned in setup.py
• Align gitignore files
• Align setup.py
• Keep the two line rule

Tests

• Make sure naming for the test files is test_.py
• Add tests_migration.py file if missing
• Remove test migration check in other files
• Check requirements.txt file for accuracy (e.g. unpin djangocms-helper)
• Check settings.py that it is ordered correctly
• Added isort and updated files

Internal

• Run internal checklist

[codecov-io]

Codecov Report

Merging #27 into master will decrease coverage by 0.14%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage      33%   32.86%   -0.15%     
==========================================
  Files          16       16              
  Lines         718      712       -6     
  Branches       71       71              
==========================================
- Hits          237      234       -3     
+ Misses        481      478       -3

Impacted Files	Coverage Δ
djangocms_history/cms_plugins.py	100% <ø> (ø)	⬆️
djangocms_history/views.py	42.1% <ø> (ø)	⬆️
djangocms_history/forms.py	100% <ø> (ø)	⬆️
djangocms_history/action_handlers.py	22.36% <ø> (ø)	⬆️
djangocms_history/operation_handlers.py	19.83% <ø> (ø)	⬆️
djangocms_history/actions.py	100% <ø> (ø)	⬆️
djangocms_history/datastructures.py	50% <ø> (ø)	⬆️
djangocms_history/compat.py	100% <ø> (ø)	⬆️
djangocms_history/signals.py	100% <ø> (ø)	⬆️
djangocms_history/helpers.py	30.76% <ø> (ø)	⬆️
... and 5 more

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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