Fix importlib.util.source_hash return annotation

[cdce8p]

Return the hash of source_bytes as bytes. [...]
https://docs.python.org/3/library/importlib.html#importlib.util.source_hash

>>> from pathlib import Path
>>> from importlib.util import source_hash
>>> 
>>> p = Path("README.md")
>>> print(source_hash(p.read_bytes()))
b'\x14\x9a~P\x1b\x89\x9f$'

Originally added in #6557

[hauntsaninja]

Thank you!

[github-actions]

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