Check md/rst in License field

[henryiii]

It looks like it might be possible to break reading by putting some types of content in License. We should either find a way to escape it, or provide a warning if it will not be readable. See python/cpython#119650.

Not as easy as I thought, as packaging can read these files that importlib.metadata can't (the PyPI description is fine).

[henryiii]

I'm not able to reproduce now.

# /// script
# dependencies = ["nox"]
# ///

import nox
import nox.__main__


@nox.session(python=["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"], venv_backend="uv")
def check(session):
    session.install("iminuit")
    session.run("python", "-c", "import importlib.metadata; importlib.metadata.metadata('iminuit')['License']")

if __name__ == "__main__":
    nox.__main__.main()

works.