mypy: python 3.10+

[oliver-sanders]

Mypy appears to be raising false positives in imported code for Python 3.10+, so currently, we're only running it for Python 3.9.

We will need to address this going forward.

The problem:

• Matplotlib has dropped Python 3.7 support in their latest releases.
• MyPy is complaining for non 3.7 compatible syntax in the non 3.7 compatible matplotlib code.
• But these non compatible versions shouldn't get installed for 3.7 anyway (the install tool should handle this).
• Adding an upper pin for Matplotlib which only applies to Python 3.7 doesn't make this go away.

Because no_silence_site_packages=false (the default) I wouldn't expect any errors from imported code anyway?

See #5735

[MetRonnie]

python/mypy#9972

I don't think we can do anything about this

[oliver-sanders]

Ok, I guess we'll stick with 3.9 for linting until we are able to drop 3.7 support. Bit of a pain for development, but easy to achieve in CI.

[oliver-sanders]

@MetRonnie, closed by #6809?