remove mistune approximate pin, add 3.0 compatibility, add 3.0 lower pin

[braingram]

The tests and the CI don't do a great job of testing these changes.

The use of mistune in sphinx-asdf is to convert markdown embedded in the ASDF schemas to rst for rendering in sphinx. This was previously handled with a custom rst renderer registered with mistune (which handled the markdown parsing). The mistune API changed significantly from 0.8 to 3.0. 3.0 includes a built in rst renderer (which this PR uses to replace the custom rst renderer). Mistune 3.0 also has a built-in math plugin. However, this is not compatible with the syntax used within the ASDF schemas. This PR uses the new mistune api and registers a custom plugin with regexes that are more similar to those used prior to this PR (using the exact regexes did not work and these needed changes to include inline flags, change group names, etc). (See related mistune math plugin issue: lepture/mistune#330).

This screenshot shows the conic schema in asdf-transform-schemas rendered with this PR. Showing both inline and block math rendering:

Note that nbconvert (which also uses mistune and is the common cause of version conflicts with sphinx-asdf) only recently allowed mistune 3.0 so some users might need to update nbconvert.
jupyter/nbconvert#1820

Fixes: https://jira.stsci.edu/browse/AL-707