Fix mypy linting issues

Fix mypy linting issues

### Description

Fixing mypy linting issues for new mypy version.

### Checklist

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: #1564
Pull-request: https://github.com/sqlalchemy/alembic/pull/1564
Pull-request-sha: a79d47d3acf7ae628ead7ca829bfe1c6f864c3c9

Change-Id: I3be88b6328dc07c46e6b1ddf8c85480dc88cfe95