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.

Edit: The pyright dependency was removed; it is not intended for this use case, per this review.

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, ...)