[rk042]

What does this change?

Adds mypy to the development dependencies and runs it against src/booth/
in a separate GitHub Actions job. The job runs on Python 3.12 while checking
compatibility with BOOTH's minimum supported Python version, Python 3.9.

Closes #17

Why?

BOOTH ships a py.typed marker, so type checkers trust its annotations.
This CI check prevents those annotations from silently becoming stale.

Checklist

• Ran the full test suite locally: 384 passed, 6 skipped.
• Ran mypy src/booth/: no issues found in 2 source files.
• No existing tests were modified; this is a CI-only change.
• No runtime behavior or runtime dependencies were changed.
• No changelog entry is needed because this does not change public behavior.
• This is not a bug fix; the issue is a CI/tooling improvement.

Anything else worth knowing?

Mypy is pinned to >=1.19,<2.0. Python 3.12 is used to run mypy in CI,
while python_version = "3.9" checks compatibility with BOOTH's minimum
supported Python version.

[Vedantgitbot]

Thanks @rk042 , nice contribution. The package builds, mypy is clean on the 3.9 target, all 397 tests pass, and the workflow is valid. Merging. I'll make a small follow-up to move mypy into its own extra.