Resolving `TypedDict` fails under Python 3.10

[Zac-HD]

The relevant tests were simply skipped for #2896, but we should return to this and get them working before 3.10 is stable; and check whether this can happen with from future import annotations on earlier versions.

The tests could also be upgraded to have non-builtin types as members of the dicts, since I'm planning some unrelated improvements for forwardrefs to builtins.

[Zac-HD]

Note that #2899 will make the skipped tests pass without fixing the underlying problem; we'll want to add new tests using TypedDicts with user-defined types for some values.

The underlying change to treating TypedDict attributes as forward-references might also be considered an upstream bug (in typing.get_type_hints()?); we should check that and report it if so.

[Zac-HD]

Upstream is deferring PEP 563 (annotations-as-strings) until at least Python 3.11, and may yet withdraw it in favor of PEP 649 (annotations-as-closures). See also: BPO-38605, BPO-43006, python/cpython#25490, and this Steering Council email announcing the deferral.

In a few weeks when we have 3.10-beta1 and a new pytest version, we'll be able to reactivate Hypothesis' 3.10 CI job and remove the version-specific logic in our tests with git grep -E "\b3.? ?10\b" -- hypothesis-python/{src,tests}.