For Python 2, we install VOEventLib from a git fork since upstream
is not Python 2-compatible.  But we don't need to do that for Python 3!

Remove usage of .keys(), .has_key(), and .iteritems(), and
account for the fact that .values() produces an iterator in Python 3.

Previously, core.vfile.VersionedFile inherited from file,
which is problematic for several reasons, but primarily because
file doesn't exist in Python 3. So we rework it to just be a
normal class with a file attribute, and add/change certain
methods accordingly.

Import version of urlencode which is packaged with Django in order
to facilitate Python 2-3 compatibility.

we need to dance between str (unicode) and bytes to talk to hashlib and StringIO at the same time

only required on python2, but this code will break on python3 anyway

Better handling of case where a certificate already exists, but is
assigned to a different user.