Python 3.5->3.7

[ssteinbach]

Move from python 3.5 to python 3.7

[ssteinbach]

See #516

[codecov-io]

Codecov Report

Merging #526 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #526   +/-   ##
======================================
  Coverage    88.8%   88.8%           
======================================
  Files          68      68           
  Lines        7399    7399           
======================================
  Hits         6571    6571           
  Misses        828     828

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[boredstiff]

Set up a pull request from me to you.

Two warnings:

First, a deprecation warning:

test_supported_suffixes_is_not_none (test_adapter_plugin.TestAdapterSuffixes) ... /Users/alex/dev/OpenTimelineIO/opentimelineio_contrib/adapters/advanced_authoring_format.py:35: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Iterable

Easily fixable, and since VFX platform won't be targeting for 3.8, probably no need to worry about it for a long while. Still might be nice to clean it up.

Second - this wouldn't pass tests (at least for me) right away.

  ======================================================================
ERROR: test_aaf_adapter (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: test_aaf_adapter
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/Users/alex/dev/OpenTimelineIO/opentimelineio_contrib/adapters/tests/test_aaf_adapter.py", line 35, in <module>
    from opentimelineio_contrib.adapters.aaf_adapter.aaf_writer import (
  File "/Users/alex/dev/OpenTimelineIO/opentimelineio_contrib/adapters/aaf_adapter/aaf_writer.py", line 30, in <module>
    import aaf2
ModuleNotFoundError: No module named 'aaf2'

I was hitting this even though pyaaf2 is installed in my venv. So to make it happy, I explicitly added pyaaf2 to tox, even though that shouldn't be necessary.

Additionally, I did a pip install with PySide 2 in the new Python37 virtual environment and view still works fine from quick tests.

Outside of that, once you accept my PR into your branch, this should be good to go, from what I can see.

[ssteinbach]

@boredstiff I merged your PR. good call!