MNT: Update pyproject.toml for better setuptools_scm support
MNT: Update pyproject.toml for better setuptools_scm support

This partly addresses #1321, by putting our setuptools_scm config in
pyproject.toml. Leave things in setup.py for now, since all of that
properly functioning requires a pretty recent toolchain.

Unidata/MetPy,Unidata/MetPy,dopplershift/MetPy,dopplershift/MetPy