Fix `SlugRelatedField.slug_field` type: cannot be `None`#830

intgr merged 2 commits intotypeddjango:mastertypeddjango/djangorestframework-stubs:masterfrom

lev-blit:bugfix/slug-field-not-nulllev-blit/djangorestframework-stubs:bugfix/slug-field-not-nullCopy head branch name to clipboard