* chore: add some incomplete types
* chore: modernize pyproject.toml
Adding more incomplete types. About 2/3 of the way through being
able to turn on the strictness flag for it.
We are pushing pyodide_build to PyPI as a Python package, but for now,
installing pyodide_build from PyPI (i.e. pip install pyodide_build`) is almost
useless because:
there are bunch of hard-coded paths (e.g. Path(__file__).parents[2]),
its dependencies are not specified in setup.cfg.
This PR is for mitigating this situation by removing hard-coded paths and
adding tests, and is also a preparation for our new CLI
(https://github.com/pyodide/pyodide/issues/1977).