Type annotations for sqlalchemy.orm.events

Type annotations for sqlalchemy.orm.events

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### Description
An attempt to annotate `lib/sqlalchemy/orm/events.py` with type hints (issue #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.
- [x] 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: #9025
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9025
Pull-request-sha: a3fd2c0c3790164c433305ccc7ac6b73e813e037

Change-Id: I0808b6485504615fa20691dc8f4631d38bc89ab3