Fix .pyi typing for asyncio

[dkang-quora]

No description provided.

[JelleZijlstra]

It might be worth running mypy's stubtest tool in CI to confirm the stubs are correct. We could also move the annotations inline instead of using .pyi files, but that might interfere with Cython.

I think I originally created the stub files because asynq at the time still supported Python 2.

[dkang-quora]

Briefly tested on the internal codebase.

stubtest produces too many errors that are unrelated to asyncio.

> stubtest asynq

...

error: asynq.decorators.__pyx_capi__ is not present in stub
Stub: in file asynq/asynq/decorators.pyi
MISSING
Runtime:
{'_debug_options': <capsule object "struct __pyx_obj_5asynq_6_debug_DebugOptions *" at 0x7f9eb376...

error: asynq.decorators.__test__ is not present in stub
Stub: in file asynq/decorators.pyi
MISSING
Runtime:
{}

error: asynq.decorators.__traceback_hide__ is not present in stub
Stub: in file asynq/decorators.pyi
MISSING
Runtime:
True

...

Found 268 errors (checked 42 modules)