stubtest: error if a function is async at runtime but not in the stub (and vice versa)

[AlexWaygood]

Description

Stubtest does not currently error if a function is async at runtime, but not in the stub (or vice versa). This PR fixes that!

This is an attempt at implementing a regression test for these typeshed PRs:

• Sync all async defs in asyncio/tasks.pyi typeshed#6783
• Use async def instead of def ... -> Awaitable in typing typeshed#7105
• Fix several methods that should be async def, but aren't typeshed#7107

cc. @hauntsaninja

Test Plan

I've added four test cases. Note that the test file needed a bit of rejigging, as Coroutine wasn't defined in the stub for typing that teststubtest was using.

The changes in this PR generate three hits when checking typeshed. I'm not sure if they're true positives or not tbh:

error: types.AsyncGeneratorType.__anext__ is a coroutine function in the stub, but not at runtime
Stub: at line 239
def (self: types.AsyncGeneratorType[_T_co`1, _T_contra`2]) -> typing.Coroutine[Any, Any, _T_co`1]
Runtime:
def (self, /)

error: types.AsyncGeneratorType.aclose is a coroutine function in the stub, but not at runtime
Stub: at line 247
def (self: types.AsyncGeneratorType[_T_co`1, _T_contra`2]) -> typing.Coroutine[Any, Any, None]
Runtime:
<method 'aclose' of 'async_generator' objects>

error: types.AsyncGeneratorType.asend is a coroutine function in the stub, but not at runtime
Stub: at line 240
def (self: types.AsyncGeneratorType[_T_co`1, _T_contra`2], _T_contra`2) -> typing.Coroutine[Any, Any, _T_co`1]
Runtime:
<method 'asend' of 'async_generator' objects>

[hauntsaninja]

Thanks!

Error message nit: maybe is an "async def" in the stub, but not at runtime could be clearer? While "coroutine function" is true to the inspect call, I think it's less obvious.

[hauntsaninja]

The hooray for the mypy_primer is cute, but it would be quite unfortunate if stubtest ever did a thing to affect mypy_primer :D

[AlexWaygood]

The hooray for the mypy_primer is cute, but it would be quite unfortunate if stubtest ever did a thing to affect mypy_primer :D

Well, obviously :P

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

Thanks! :D