mdx_gist: inherit from InlineProcessor, work on Python 3.11 (#3630)

[AdamWill]

The gist-rst pattern crashes Python 3.11, because its pattern tries to set the multiline flag - it starts with (?m) - but Markdown's Pattern class __init__, which is called by our GistPattern.__init__, 'wraps' the pattern with additional capture groups before compiling it. This causes the multiline flag not to be at the start of the 'wrapped' pattern. From Python 3.6 to Python 3.10 flags not being at the start of the pattern was deprecated and triggered a warning when trying to compile it; in Python 3.11 it's an error.

Markdown 3.0.0 added a preferred InlineProcessor class: https://github.com/Python-Markdown/markdown/pull/629 which does not do this wrapping. It requires the subclass to implement handleMatch, which we already do. Our handleMatch uses named capture groups, so the change in the number of capture groups in the compiled expression shouldn't matter. So simply switching to inheriting from InlineProcessor instead of Pattern should solve the problem.

We already required Markdown 3.0.0, so this does not mean we require a newer Markdown than before.

Signed-off-by: Adam Williamson awilliam@redhat.com

Pull Request Checklist

• [x] I’ve read the guidelines for contributing.
• [-] I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• [x] I tested my changes.

Description

Change seems fairly trivial and should not be visible to users, so I didn't bother with AUTHORS and CHANGES. I've 'tested' this insofar as nikola now at least passes its own test suite with Python 3.11 (it did not before); the test suite does not seem to cover this file, so I don't think we'll know if it's broken, but I don't think it should be.

[Kwpolska]

Thanks for contributing this fix!