Enforce black via pre-commit tool

[dwinston]

For #33.

On git commit and git push, pre-commit runs black over relevant changed files. The first time pre-commit is run, it installs black in a separate, sandboxed Python environment that it uses on subsequent runs.

Add pre-commit install line to README, which installs hooks (from ./pre-commit-config.yaml) into .git/hooks/pre-commit so that e.g. black runs automatically on git commit.

[ml-evs]

Looks cool, I'll test tomorrow and review

[ltalirz]

thanks @dwinston !

One comment regarding python version, otherwise good.

By the way did you pre-commit run --all-files ?
Depending on how strict we want to be, you can also add something like pre-commit run --all-files || ( git status --short ; git diff ; exit 1 ) on travis

[dwinston]

@ltalirz I also just added a commit with the changes suggested by pre-commit run --all-files.

[ml-evs]

LGTM, agree that there's no reason not to move to Python 3.7 since we're using typing so heavily.

[ltalirz]

thanks @dwinston !