Fixing mistune dependency

[Mulugruntz]

Description

This is a follow-up to #13.
Full description of the issue: https://github.com/amyreese/sphinx-mdinclude/issues/12
The fix presented here is to restrict mistune dependency to only 2.* and not just >=2.

Fixes: #12

Changes made

• mistune dependency is now 2.*;

[Mulugruntz]

Let's break this out into separate PR's for each piece:

• re mistune, is there an easy way to support both major versions at once, rather than pinning to an old version? I'd rather have some if/else or try/except imports than limit to an older version unnecessarily.
• re changelog: this is generated by another project, attribution based on git tags, so any changes here will get clobbered at the next release. I'm happy to accept a PR to attribution that improves the output format.
• re version: this is also managed by attribution, so please do not change the version file directly. Perhaps attribution could add a simple comment explaining that.

Originally posted by @amyreese in https://github.com/amyreese/sphinx-mdinclude/issues/13#issuecomment-1241432354

Thanks a lot for you super quick reply :-).

What I would suggest is to have the mistune fix in 2 steps:

1. pinning (immediate fix). <-- this is what this PR does.
2. extending the support and enlarging the deps.
   • I can open a new issue to keep track of that future follow-up.

About the changelog, I'll see if I can quickly submit a fix to attribution :-).