[PR #6923/78fa0401 backport][3.8] Exclude the object possibility from the ClientSession.timeout type

[patchback]

This is a backport of PR #6923 as merged into master (78fa040).

What do these changes do?

This is a follow-up PR to #6917, which fixes the overly inclusive typing of ClientSession.timeout attribute, which can never actually end up as an object instance. This leftover probably exists from the time when there was the _sentinel value being assigned to the attribute (see "3.8 branch self._timeout assignment" from the linked issue), hence why it ended up this way.

Are there changes in behavior for the user?

For untyped code, there's absolutely no change - the self._timeout attribute was never actually assigned the sentinel value to. Most of the explanation already exists in #6917, here's the test code snippet from there again:

import asyncio
import aiohttp

async def main:
    session = aiohttp.ClientSession(timeout=aiohttp.ClientTimeout(total=10))
    # read back the total time attribute
    total_time = session.timeout.total  # "object" type of "Union[object, ClientTimeout]" has no attribute "total"
    print(total_time)

asyncio.run(main())

For typed code, the current solution would be to assert isinstance(session.timeout, aiohttp.ClientTimeout) everywhere the attribute is being accessed. This PR removes this "unnecessary necessity".

Related issue number

#6917

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes (documentation never mentioned an object instance being accessible from there even)
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