Fix `RowMapping`'s `Mapping` type to reflect that it supports `Column`s or strings

Fix `RowMapping`'s `Mapping` type to reflect that it supports `Column`s or strings

### Description

I ran into this originally in sqlalchemy2-stubs: https://github.com/sqlalchemy/sqlalchemy2-stubs/pull/251, where `RowMapping` only supported string keys according to the type hints. I ran into a similar issue here upgrading our application where because `RowMapping` subclassed `Mapping[str, Any]`, `Row._mapping.get()` would fail to typecheck when used with `Column` objects.

This patch adds a test to verify that `Row._mapping.get()` continues to work with both strings and `Column`s, though it doesn't look like mypy checks types in the tests.

Fixes #9644.

### Checklist
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This pull request is:

- [ ] A documentation / typographical 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.

**Have a nice day!**

Closes: #9643
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9643
Pull-request-sha: 6c33fe534cf457d6b5c73f4830a64880830f0f56

Change-Id: I1009c6defff109d73f13a9e8c51641009e6a79e2