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.
Differential Revision: https://phab.mercurial-scm.org/D9833
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif