Addon code alignments

[FinalAngel]

• Extended test matrix
• Added tox and adapted imports
• Adapted code base to align with other supported addons

General

• [x] Update changelog Adapted code base to align with other supported addons
• [x] 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].
• [x] Check that python, django, django-cms and other versions are listed correctly (add badge)
• [x] Make sure Django is not pinned in setup.py
• [x] Align gitignore files
• [x] Align setup.py
• [x] Keep the two line rule

Tests

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

Internal

• [x] Run internal checklist

[codecov-io]

Codecov Report

Merging #6 into master will not change coverage. The diff coverage is 61.11%.

@@           Coverage Diff           @@
##           master       #6   +/-   ##
=======================================
  Coverage   30.64%   30.64%           
=======================================
  Files          10       10           
  Lines         372      372           
  Branches       56       56           
=======================================
  Hits          114      114           
  Misses        258      258

Impacted Files	Coverage Δ
djangocms_transfer/apps.py	100% <ø> (ø)	:arrow_up:
djangocms_transfer/importer.py	21.05% <ø> (ø)	:arrow_up:
djangocms_transfer/datastructures.py	48.38% <ø> (ø)	:arrow_up:
djangocms_transfer/helpers.py	20.68% <ø> (ø)	:arrow_up:
djangocms_transfer/compat.py	100% <ø> (ø)	:arrow_up:
djangocms_transfer/cms_toolbars.py	0% <0%> (ø)	:arrow_up:
djangocms_transfer/utils.py	36.84% <100%> (ø)	:arrow_up:
djangocms_transfer/exporter.py	37.14% <100%> (ø)	:arrow_up:
djangocms_transfer/forms.py	25.77% <42.85%> (+0.77%)	:arrow_up:
djangocms_transfer/cms_plugins.py	34.14% <75%> (-0.8%)	:arrow_down:

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