* 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.
Also used the "slim" version, which will drastically reduce the
image size.
Base image sizes comparison:
| python:3.7-slim | 143MB |
| python:3.7 | 916MB |
