Remove redundant code for EOL Python <= 3.6

Remove redundant code for EOL Python <= 3.6

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### Description
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There's a few bits and pieces of code to support Python <= 3.6 which are no longer needed and can be removed, to slightly simplify the codebase.

### 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: #7544
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7544
Pull-request-sha: 282b4a91282902a57807aa2541b75b272b547127

Change-Id: I9ddf15fcf72551d52e3f027f337c7fee4aa9083b