Mock pytz.timezone localize in tests.
Ref: #4986

Ref: #4986

Fix bug for not existent or ambiguous datetime during native to aware conversion in timezone with DST.
Ref: #4986

Added test for DateTimeField validation when server has timezone with DST and input is a native time in a DST shift interval.
Added pytz to requirements-testing.txt to reproduce the case.

Support HEAD in viewsets

Use 'items' templatetag throughout.

Updated funding page

Support usage of 'source' in `extra_kwargs`.

Update coreapi.js version in JavaScript API docs.

* Fixes #4950.

* converted testing test asserts to pytest

* moved 3rd arg from test_empty_post_uses_default_boolean_value test assert

fixed issue on schemas.py where if model._meta.get_field(variable) failed model_field was not set to None and would reult in an error

* Default value will now be used when serializing if key or attribute is missing.

* directly using Django's JSONResponse

* fixed JsonResponse typo

* added safe=False to JsonResponse

* removed unnecessary 'safe=False' in JsonResponse