Fix return type of asyncio.StreamReader.exception

[rijenkii]

According to typeshed, function asyncio.StreamReader.exception returns Exception. This is provably incorrect:

asyncio REPL 3.14.4 (main, Apr 16 2026, 00:00:00) [GCC 15.2.1 20260123 (Red Hat 15.2.1-7)] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> reader = asyncio.StreamReader()
>>> type(reader.exception())
<class 'NoneType'>

This PR fixes return type of asyncio.StreamReader.exception, making it a union of Exception and None.

[github-actions]

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