Support python3.10

[sobolevn]

Right now typecheck tests fail with:

drf_source/tests/test_permissions.py:548: error: "object" not callable
drf_source/tests/test_permissions.py:554: error: "object" not callable
drf_source/tests/test_permissions.py:608: error: "object" not callable
drf_source/tests/test_permissions.py:615: error: Cannot assign multiple types to name "composed_perm" without an explicit "Type[...]" annotation
drf_source/tests/test_permissions.py:628: error: "object" not callable
drf_source/tests/test_permissions.py:635: error: Cannot assign multiple types to name "composed_perm" without an explicit "Type[...]" annotation

Link: https://github.com/typeddjango/djangorestframework-stubs/runs/7000977052?check_suite_focus=true#step:6:421
Source: https://github.com/encode/django-rest-framework/blob/fa9d516ee24b2447a351af92e139d386b4f0d2b4/tests/test_permissions.py#L548
Initial attempt: #236

[intgr]

Fixed in #292.