MNT: migrate requirement files to PEP 735 dependency groups

[neutrinoceros]

No description provided.

[neutrinoceros]

Run uv run --no-editable --compile-bytecode --group test --no-group typecheck --resolution=lowest-direct --exclude-newer '2022-03-27' \
  × No solution found when resolving dependencies:
  ╰─▶ Because cmyt:typecheck depends on matplotlib>=3.8 and only
      matplotlib<3.8 is available, we can conclude that cmyt:typecheck is
      incompatible.
      And because your project depends on cmyt:typecheck, we can conclude that
      your project's requirements are unsatisfiable.

      hint: Pre-releases are available for matplotlib in the requested
      range (e.g., 3.9.0rc2), but pre-releases weren't enabled (try:
      `--prerelease=allow`)

looks like a bug upstream in uv. Will report later.

[neutrinoceros]

Hum actually, thinking a bit more about this, it doesn't seem like a bug in uv: uv run runs uv sync which always needs to look at all dependencies (extras an groups included) to ensure that the resolved environment stays compatible with a global lock file.
I see two ways to go about it:

• keep a requirement file for typechecking for now
• bite the bullet now and require matplotlib >= 3.8. This may feel a bit radical now but at the same time this package hasn't needed a release in a year and probably won't for the foreseeable future, so I think I'll run with it.