Make this library compatible with 3.8+

[InSyncWithFoo]

Since the main use of this library seems to be type checking, I think it's a good thing to make this library also usable in 3.8 and 3.9.

[Tinche]

The tests are failing because signature doesn't allow eval_str on older Pythons. I guess that needs to be handled, and those versions can't support from __future__ import annotations properly I think.

[InSyncWithFoo]

That could potentially be replaced with typing.get_type_hints() (or, to be precise, typing_extensions.get_type_hints()). According to this CPython issue, there are subtle differences, but I think it's good enough. Should I proceed?

[Tinche]

Sure, give it a try!

[InSyncWithFoo]

That was actually more complex than I thought. Anyway, tests pass now.

[Tinche]

Thanks!