* Remove Travis Python 3.7 hack
No longer necessary: 3.7 is supported out-of-the-box
* Add support for Python 3.8
And explicitly document in setup.py
* "Upgrade base Docker image to Python 3.8"a
* Add entry about py38 support in WIP changelog
* Explicitly declare python versions in setup.py
* Bump Werkzeug to 0.15.6
This is the latest 0.15 version. 0.16 might be incompatible
The fix we are looking for is in [0.15.5](http://werkzeug.palletsprojects.com/en/0.15.x/changes/#version-0-15-5):
> Fix a TypeError due to changes to ast.Module in Python 3.8.
- "{}".format('foo') does not work on python2.6, as the index needs to be explicitly specified.
- assertIn(x, y) was only introduced in 2.7, reverting to assertTrue(x in y)
Updated test environments definitions and docs accordingly.
With only 3.5, travis-ci works, because 2.7, 3.3 and 3.4 interpreters
are present by default, but it might break randomly at some point.
Definining all the versions explicitly, and using the tox-travis plugin
is more robust.
Also, 2.6 was removed from the "officially supported" python versions,
as the tests are not run (and don't pass).
