Add typing overloads to Query.__getitem__ and AppenderQuery.__getitem__

Add typing overloads to Query.__getitem__ and AppenderQuery.__getitem__

Fixes #13128

### Description
`Query.__getitem__` and `AppenderQuery.__getitem__` previously returned Union[_T, List[_T]] for all inputs, making the return type inaccurate.
Added `@overload` signatures so that integer index returns _T and slice returns List[_T].

This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed

Closes: #13142
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13142
Pull-request-sha: 9ba1f0145d90b18c997137aeba7fde72dac23a7c

Change-Id: Ib37ab63d3d844491c34cc5ccfc4efc1591a1878c
(cherry picked from commit 8af25c2b35f3740a00e2551c65f9ba245e6f5a9d)