bug#22437: Problems with non-auto-translated python-2 package variants

[Leo Famulari]

Python-2 variants of python-3 packages that are not automatically
translated by package-with-python2 are not used by auto-translated
python-2 packages.

For example, python2-cryptography requires python2-ipaddress. This is
not required by python-cryptography:
http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/python.scm#n5838

Python-oauthlib requires python-cryptography, and thus cannot be
translated wholly by package-with-python2. Package-with-python2 fails to
refer to the "hand-made" python2-cryptography that includes
python2-ipaddress, and insteads attempts to generate it automatically.
This results in a failed build of python2-cryptopgraphy because
python2-ipaddress is not being provided. The solution is to remove
python-cryptography from python2-oauthlib and provide
python2-cryptography "by hand":
http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/python.scm#n2333

This is especially annoying when a python-2 variant needs to refer to
more than one of these "hand-made" python-2 packages:
http://lists.gnu.org/archive/html/guix-devel/2015-12/msg00921.html

There are many python-3 packages that include python-setuptools, even
though they build without it. I assume this is to easily take advantage
of package-with-python2.

Some context:
bug: python2 versions of packages failing as inputs
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22013
guix-devel: Python2-jsonschema fails:
http://lists.gnu.org/archive/html/guix-devel/2016-01/msg00604.html
guix-devel: Split acme into both Python variants
http://lists.gnu.org/archive/html/guix-devel/2015-12/msg00921.html