Add python 3.7 to nox testing and to setup.py

[jtmiclat]

Add pythjon 3.7 to nox testing and to setup.py

Caveat. Flask running in python 3.7 emits a warning message

.nox/tests-3-7/lib/python3.7/site-packages/jinja2/utils.py:485
  /app/.nox/tests-3-7/lib/python3.7/site-packages/jinja2/utils.py:485: 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 MutableMapping

.nox/tests-3-7/lib/python3.7/site-packages/jinja2/runtime.py:318
  /app/.nox/tests-3-7/lib/python3.7/site-packages/jinja2/runtime.py:318: 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 Mapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html

[davidism]

The deprecation warnings were addressed in Jinja 2.10.2, released 2019-10-04.