Relax the Python version check for `__class_getitem__` tests

Relax the Python version check for `__class_getitem__` tests

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### Description
`__class_getitem__` to support generics was introduced in Python 3.7.
In 3.9 some built-ins were made generic but the functionality
for user-defined classes has been there since 3.7.

### 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: #7049
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7049
Pull-request-sha: 7bd16d416db010e7140313fe326861cc263b8864

Change-Id: I4f45b9585d263636e613cc5c86e710fa31c7d283