Python Support

We've recently had an issue where the twine package in fedora rawhide [0] stopped providing python2-twine.

The maintainer for twine stated that he would "look at building a python2-twine package again that doesn't provide any binaries" [1] This would resolves our problem for the short term (until f28 is released in May 2018). But in the long run we should remove our dependency on twine.

We should refactor our pulp-python Package model [2] to use pkg_resources and pkginfo to parse package info, since twine is not meant to be a library and can change its internal APIs without warning.

[0] https://apps.fedoraproject.org/packages/python-twine
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1512552#c3
[2] https://github.com/pulp/pulp_python/blob/77ec66251cc12e8b4def37c30c0381a64d97690e/plugins/pulp_python/plugins/models.py#L138