CI: Add typecheck suppressions due to django-stubs changes

[intgr]

Fixes new errors that appeared after django-stubs changes.

drf_source/tests/test_response.py:259: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
drf_source/tests/test_response.py:259: note: Consider using "Mapping" instead, which is covariant in the value type
drf_source/tests/test_response.py:269: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
drf_source/tests/test_response.py:269: note: Consider using "Mapping" instead, which is covariant in the value type
drf_source/tests/test_response.py:278: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
drf_source/tests/test_response.py:278: note: Consider using "Mapping" instead, which is covariant in the value type

Related issues

Probably caused by typeddjango/django-stubs#1529

[sobolevn]

I think that we can use Mapping there, the only method that is using headers is https://github.com/django/django/blob/4f6a51dfe6a4a8d5ed61c73b902e808109f142b0/django/http/request.py#L461

And it allows Mapping.

[intgr]

Fixed in typeddjango/django-stubs#1537 instead, closing.