build: use meson-python

[alexfikl]

This essentially ports PyWENO to use meson-python https://meson-python.readthedocs.io to build instead of setuptools. I went with meson-python because that seems to be what numpy, scipy and matplotlib are using these days, so it's fairly well-supported.

Some additional non-trivial changes:

• Using the modern pyproject.toml way to configure things: setup.py is essentially a stub now.
• Removed version and git_version: it now relies on https://docs.python.org/3/library/importlib.metadata.html to get version information.
• Switched the docs to use the modern sphinx_rtd_theme instead so that it looks less old :grin:

Ran the tests and the examples and everything seems to be working fine, but it may need a careful look so it doesn't just "work on my machine".

I know PyWENO is mostly dead, unfortunately, but I was using it to double-check another implementation and thought it would be useful fix this upstream. It was actually failing a build step because it wasn't declaring setup_requires=["numpy"] or something like that.