MyPy Support

[acederberg]

Hi!

I am checking my code with mypy and would like to ask that we somehow add support. When I run mypy on my filters I get the following message:

scripts/filters/util.py:7: error: Skipping analyzing "panflute": module is installed, but missing library stubs or py.typed marker  [import-untyped]

This will not take much (so I suppose) and the work involved is pretty minimal.
Let me know if there are any issues with this and eventually I might get around to making a PR about it.

[acederberg]

For now, I am doing

[tool.mypy]
overrides = [
  {module = "panflute.*",ignore_missing_imports=true},
]

in pyproject.toml to suppress these errors.