Add optional dev dependency

[Soheab]

Summary

I noticed that discord.py doesn't have a optional list of dependencies needed to develop the library. So here it is.

Black and Pyright versions are straight from the lint workflow.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[AbstractUmbra]

Might be a questionable addition since pyright itself is actually an npm package. The python module uses nodeenv which by the authors own admission is less stable than using pyright[nodejs] which uses nodejs-wheel in its stead.

My suggestion would be omitting this entirely and letting the users use Pylance or their own installation of pyright.