Use voeventparse instead of VOEventLib

This is necessary since we use --system-site-packages to create
the virtualenv, and the system version of numpy is too old.

* Switch from VOEventLib to voevent-parse due to significant
  string-bytes issues in VOEventLib
* Remove bad exception handling in events API for creating a
  VOEvent
* Add parameter checking to events API for creating a VOEvent
* Combine VOEvent building code for events and superevents into
  a single instance which handles both cases

Also, use Python 3.5 instead of Python 3.7 - I think we're going
to be using 3.5 for a bit since that's default for Debian 9.

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