Move SetupTools metadata into pyproject.toml

SetupTools 61 adds experimental support for reading metadata from
pyproject.toml files instead of traditional setup.py or setup.cfg.
PBR still seems to require the package name in setup.cfg for now,
but otherwise hollow this file out moving everything to the new
solution. Because this is experimental, silence a warning from
SetupTools which states that. Also work around newer SetupTools
complaints about namespace packages included as data by informing it
that mudpy.tests and mudpy.tests.fixtures are included modules even
though they really aren't.