feat: switch the build-backend to pdm-backend

[onerandomusername]

Summary

Finally drop setuptools, closes https://github.com/DisnakeDev/disnake/issues/664

Checklist

• [x] If code changes were made, then they have been tested
  • [x] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running pdm lint
  • [x] I have type-checked the code by running pdm pyright
• [x] 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, ...)

[onerandomusername]

Moved the dynamic versioning aspect to #1007, so this is now free to be reviewed and/or merged at any time.

[shiftinv]

@onerandomusername any updates? Would be cool to merge this sooner rather than later c: