Hello, this issue is a byproduct of issue42195. It has already been fixed on Python 3.10, and on Python 3.9.2 (which isn't out yet). You can see the what's new for it here https://docs.python.org/3/whatsnew/3.9.html#notable-changes-in-python-3-9-2.

The expected release date for Python 3.9.2 is Monday, 2021-02-15 according to PEP 596 https://www.python.org/dev/peps/pep-0596/.

For now, I guess you'll have to use the old typing.Callable, then update it in newer versions of Python.

On Python 3.10a4:
>>> from typing import Optional
>>> from collections.abc import Callable
>>> Optional[Callable[[bytes], bytes]]
typing.Optional[collections.abc.Callable[[bytes], bytes]]