dev-python/numpy-python2: Package supporting py2 for smoother transition

dev-python/numpy-python2: Package supporting py2 for smoother transition

As discussed in https://bugs.gentoo.org/703240 I will maintain a
numpy-python2 package to allow easier transition of reverse deps allowing to
still port them to python 3.8.

The idea of this package is to drop it as soon as possible, as a
consequence:
- python2-only packages depending on this should have a bug opened and
blocking bug #703754 to allow us to track upstream progress on python3
porting. As a consequence, if the package upstream is dead or not planning
to port to python3 ever, it should be treecleaned instead of adding a
dependency on this package.
- python3 packages also needing python2 support. During a transition period,
it's possible that we will need to temporally allow some packages (usually
libraries) to support python2 until other reverse deps of that libs are
handled. You should then open a bug report blocking bug #703756 to follow
the progress on porting them completely to python3. As a consequence, if
your package supports both, you must try to simply remove python2 support
over depending on this package, while only adding a dep on numpy-python2 if
it is really hard to handle all reverse deps immediately. In that case, a
bug report need to be opened as explained.

Closes: https://bugs.gentoo.org/703240
Package-Manager: Portage-2.3.82, Repoman-2.3.20
Signed-off-by: Pacho Ramos <pacho@gentoo.org>