Introduce tox

[kurtmckee]

This PR introduces a tox configuration that standardizes all of the project testing using tox.

It does not eliminate the Makefile, but pushes all of the work into the tox configuration so that all platforms (whether Linux, macOS, or Windows) and runner environments (whether locally or in CI) are running the test suite in the same way.

Commit highlights

• Add pyright as a dev dependency.

• Add tox configurations that run pyright and mkdocs on Python 3.10.

  This matches the Python version currently used in CI.

• Add a tox config to run the test suite against Python 3.10+.

• Fix a CLI rendering test on Windows.

• Update CI to install tox and tox-uv.

• Use tox to execute the test suite in CI.

  This ensures that CI and local testing mirror each other.

Decisions made

• tox-uv (and thus, tox) is not added as a dev dependency.

  tox-uv depends on uv, which would add uv
  as a locked dependency in uv.lock.
  This seems circular, so tox-uv is not a dev dependency.

• The mkdocs --strict mode appears to work similar to
  the existing architecture in CI, which greps CLI output
  for the would "ERROR".

  However, it may be that --strict mode fails fast,
  so this might not be strictly equivalent to output grepping.

• Coverage files are no longer stored in coverage/;
  they are instead stored in .coverage* files.

  This is not a deliberate decision, but a side-effect.

Closes #234

[zmievsa]

Hi! Thanks for publishing! Still looking into it but can't guarantee when I'll be able to get to it due to holidays.