Skip mypy plugin tests if incompatible or missing

Skip mypy plugin tests if incompatible or missing

Fixes: #12287
<!-- Provide a general summary of your proposed changes in the Title field above -->

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

This skips Mypy plugin tests if mypy is missing or an unsupported version.

### 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 / small typing 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: #12288
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12288
Pull-request-sha: 00e00f321d54da3e5d6112f61d5770e59e33bf82

Change-Id: I492a93d3c586425e2cf53304520164dc1487a667