Type annotations for sqlalchemy.orm.mapped_collection

Type annotations for sqlalchemy.orm.mapped_collection

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### Description
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An attempt to annotate lib/sqlalchemy/orm/mapped_collection.py with type hints (issue https://github.com/sqlalchemy/sqlalchemy/issues/6810)

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [ ] 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: #9140
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9140
Pull-request-sha: facb4717134943dd651905f7c72618eb66a9eca5

Change-Id: I0fb80e2ea7ed2247c494487fb6c8d72efb4e9802