Pin black version

[dstansby]

PR summary

Each calendar year black can make updates/changes to how they format code (see the black stability policy). To make sure our autogenerated pyplot.py corresponds to a particular version of black, add a pin to the version. When we want to update we can then bump the major version of black and regenerate pyplot.py in sync.

Fixes partially #27529 - I'm not sure if there's a way to run the weekly GH actions workflows on a PR to check this works?

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[tacaswell]

The test failure is a wx memory test that is also failing on main