pyproject: add config file
This will tell pip et al to call our setup.py for the majority of
packaging concerns, but also gives us a place to put standard config
stuff like black.
This was previously D9833, but was rolled back due to test
breakage. nbjoerg thinks that breakage is now resolved, so we're
trying again.
Differential Revision: https://phab.mercurial-scm.org/D10184
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL