Fixes: #10673: make declared_attr covariant

Fixes: #10673: make declared_attr covariant
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### Description
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I made declared_attr covariant as suggested in #10673. mypy didn't seem to complain. Added a regression test for the use case that was asked for. Unfortunately, it seems like using `Mapped[int | UUID]` directly in the Protocol won't work:

```python
class CompareProtocol(Protocol):
    id: Mapped[int | UUID]
```

Because mypy will see this as a settable variable and not as a SQLAlchemy descriptor. Using `@property` instead seems to work and it's what I used in the test (perhaps it should be documented as the way to achieve this?):

```python
class CompareProtocol(Protocol):
    @property
    def id(self) -> Mapped[int | UUID]: ...
```

### Checklist
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This pull request is:

- [ ] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [X] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

Closes: #13266
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13266
Pull-request-sha: 8edd2841f4bbe61f8bb9bc15a7a57e0560698779

Change-Id: I7d63ad43df0ab34ee7c7389a007191be91efa574
(cherry picked from commit f1dfe10237d754ae25b9790c9a9e4d9defccb52b)