If you'd like to contribute to Mastodon.py, here's some concrete areas that need work:

Features are currently up to date and mostly tested, but there are some areas where tests could be better, especially for all the wrong-argument and error state checks. Check the codecov page and try to add tests those areas.
Nodeinfo is currently just documented by reference to the spec. It could be documented better.
Nodeinfo currently just retrieves the 2.0 spec version. It should likely attempt to retrieve other versions as well, trying to get the most recent one available.
We currently don't use OAuth Proof Key for Code Exchange - that should be implemented at some point (for servers that support it).
Other implementations of the Mastodon API as well as Mastodon forks exist - it may be good to try to support these:
- Figure out what they do different and file issues / document it
- Where code can be written to support alternate implementations or different features, write code to do this
- Write tests specific to these features
- We now have a "feature set" parameter to support these better.
You can also check TODO.md for any open TODOs that have been added.

General guidelines

If you've discovered what you think is a bug, or are even just unclear on whether something is a bug, or about how to do some specific thing - just open an issue, we'll try to respond.
If you're contributing code, while it'd be great if you could provide tests, if setting up a mastodon server to test against is too much, feel free to go without - we can always add those later.
There is a .githooks directory that you can start using by running "$ git config --local core.hooksPath .githooks/". Right now, there is only a hook that checks whether you're trying to commit a credential.
We don't generally accept purely cosmetic changes, please avoid patches that only apply formatting.
If you have any questions about contributing or the library, you can either open up a discussion in the github discussions section, open an issue, or directly talk to @halcy@icosahedron.website on the Fediverse or .halcy on discord.