Fix PyRight type checking

[maxfischer2781]

The regular static checks action recently started failing (e.g. action 730) due to a PyRight type checking issue:

pyright 1.1.392, node v20.18.0, pyright-action 2.3.2
/home/runner/work/asyncstdlib/asyncstdlib/asyncstdlib/functools.py:273:32 - error: Argument type is partially unknown
  Argument corresponds to parameter "func" in function "iscoroutinefunction"
  Argument type is "((T@cached_property) -> Awaitable[R@cached_property]) | type[Unknown]" (reportUnknownArgumentType)
1 error, 0 warnings, 0 informations
Error: 1 error