py-gobject3, meson, and python versioning

[nia <nia%NetBSD.org@localhost>]

The latest py-gobject3 update broke the py27 version and a bunch of
dependent packages. This is because it now needs meson and pkgsrc can't
invoke meson when python2 is in use by the package - it's python3 only.

Non-pkgsrc projects can still successfully build the py27 version of
py-gobject3 by invoking a python3-using meson with
-Dpython=${PREFIX}/bin/python2. 

This seems like a problem with how meson is used in pkgsrc. It should
be treated like a tool rather than a normal python module, probably
not be version prefixed, and not set PYTHON_VERSIONS_INCOMPATIBLE for
packages using it.

Basically, meson's use of python3 shouldn't be affecting packages using
it as a build system.

Alternatively: we keep the pre-meson py-gobject3, or we delete all the
Python 2 packages using py-gobject3.