Exclude the object possibility from the ClientSession.timeout type

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.

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:

```py
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".

Co-authored-by: Sviatoslav Sydorenko <wk@sydorenko.org.ua>

Fixes #6917
PR #6923

(cherry picked from commit 78fa040)

CHANGES/6917.bugfix

@@ -0,0 +1,3 @@
+Dropped the :class:`object` type possibility from
+the :py:attr:`aiohttp.ClientSession.timeout`
+property return type declaration.

CHANGES/6917.doc

@@ -0,0 +1 @@
+6917.bugfix

CHANGES/6923.bugfix

@@ -0,0 +1 @@
+6917.bugfix

CHANGES/6923.doc

@@ -0,0 +1 @@
+6917.doc

aiohttp/client.py

@@ -1027,7 +1027,7 @@ def loop(self) -> asyncio.AbstractEventLoop:
         return self._loop
 
     @property
-    def timeout(self) -> Union[object, ClientTimeout]:
+    def timeout(self) -> ClientTimeout:
         """Timeout for the session."""
         return self._timeout