There were some copy/paste errors that I cleaned up. Interestingly though,
voeventlib-parse will just do crazy things with the xml nesting instead
of throwing an error.

added em_type and coinc_far to api requests. next step: modify client to update these fields, make sure logging and alerts are active, and then modify tests.

    - 0052_update_gstlalcbc_O3b_cert.py: gstlalcbc was issued a new cert at CIT.
      I manually migrated on production and playground, but i'm backing up the
      migration here.
    - 0005_add_coinc_far_and_em_type.py: snapshotting the migration for the new
      `em_coinc` and `em_type` data fields. This will most likely change.

Not sure why these were generated. The models were not modified,
but maybe it was just because we are now using Python 3, and string
handling has changed compared to Python 2.  I think all of the fields
that were touched have strings in them either as a default option
or as a set of choices.

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!