Relax future() and future_safe() argument types from Coroutine to Awaitable

[syzayd]

I have made things!

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

Closes #2404

What changed

future() and future_safe() typed their wrapped function argument as returning Coroutine, which is stricter than needed. A callable typed as Callable[..., Awaitable[T]] (for example an async def that awaits and returns some other awaitable, rather than one whose own return is annotated Coroutine) failed type checking even though the decorators only ever await the result at runtime.

This widens the parameter type on both decorators (and the internal _future_safe_factory helper) to Awaitable, and removes the now unused _SecondType TypeVar that only existed to parameterize Coroutine.

Test plan

• Added a typesafety case to test_future_decorator.yml and test_future_safe_decorator.yml covering a Callable[[int], Awaitable[int]] argument that is not itself typed as Coroutine.
• pytest typesafety/test_future (53 passed)
• pytest tests -k future (133 passed)
• mypy returns (no issues)
• ruff check returns/future.py typesafety/test_future (all checks passed)