Add support for Python 3.10

Add support for Python 3.10

<!-- Provide a general summary of your proposed changes in the Title field above -->

### Description
<!-- Describe your changes in detail -->

Python 3.10 was released last week.

Test on Python 3.10 and add the Trove classifier.

### Checklist
<!-- go over following points. check them with an `x` if they do apply, (they turn into clickable checkboxes once the PR is submitted, so no need to do everything at once)

-->

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!**

^ Which checklist option should I use?

Should I do something to trigger/test all the workflows?

Closes: #7179
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7179
Pull-request-sha: 5df7ff0e0d309301a877314e620fadb6f3f570cb

Change-Id: Ib7ecc0de75f7dffdf05db13563f7be10898e67ba