Release/3.0.0

[tomwojcik]

todos:

• update docs
• update comments / docstrings, if needed
• move tests to upstream so the diff is smaller and more reliable (unused mocks etc)
• some tests are mediocre at best. Compare the copy-pasted post methods body vs upstream ModelViewSet actions
• in the upstream add tests to check the resolved view paths to guarantee they did not change after the migration

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.37695% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.57%. Comparing base (4d296cf) to head (26a5383). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
djoser/views/user/me.py	84.61%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #820 +/- ## ========================================== - Coverage 98.67% 98.57% -0.10% ========================================== Files 27 39 +12 Lines 903 981 +78 ========================================== + Hits 891 967 +76 - Misses 12 14 +2 ```

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