I've run the black code formatting tool against everything:
black tests datasette setup.py
I also added a new unit test, in tests/test_black.py, which will fail if the code does not
conform to black's exacting standards.
This unit test only runs on Python 3.6 or higher, because black itself doesn't run on 3.5.
Means `datasette publish heroku` can work under Travis, unlike this failure:
https://travis-ci.org/simonw/fivethirtyeight-datasette/builds/488047550
```
2.25s$ datasette publish heroku fivethirtyeight.db -m metadata.json -n fivethirtyeight-datasette
tar: unrecognized option '--exclude-vcs-ignores'
Try 'tar --help' or 'tar --usage' for more information.
▸ Command failed: tar cz -C /tmp/tmpuaxm7i8f --exclude-vcs-ignores --exclude
▸ .git --exclude .gitmodules . >
▸ /tmp/f49440e0-1bf3-4d3f-9eb0-fbc2967d1fd4.tar.gz
▸ tar: unrecognized option '--exclude-vcs-ignores'
▸ Try 'tar --help' or 'tar --usage' for more information.
▸
The command "datasette publish heroku fivethirtyeight.db -m metadata.json -n fivethirtyeight-datasette" exited with 0.
```
The fix for that issue is to call the heroku command like this:
heroku builds:create -a app_name --include-vcs-ignore
