Python dependencies for ports requiring Python 3

[Raphael Kubo da Costa]

Hi, everyone,

I've been helping get archivers/py-borgbackup (bug 207715) committed for
a while now.

The last remaining blocker is the fact that the port has
USES=python:3.4+ and depends on other Python ports, namely
devel/py-setuptools_scm, devel/py-msgpack-python and devel/py-llfuse.
This means the port will try to require/install the Python 3 versions of
those ports which are not built by default and not present in Poudriere
builds if one does not explicitly build Python 3 packages.

I'd like to confirm if the suggestion in comment #2 in that bug is the
only possible approach: doing what was done to devel/py-setuptools and
creating version-specific ports for both py-setuptools_scm and
py-msgpack-python. In practice this means adding 12 new ports to the
tree (and coordinating with the current maintainers), and the problem
will happen again if py-borgbackup introduces any new Python
dependencies in future releases.