Add standardized code formatting

[holmbuar]

In my fork of this repo, I prefer to apply code formatting. In particular I like to break up long code lines, to avoid horizontal scrolling and increase readability.

My preferred Python formatter is Black, one of many available.

https://pypi.org/project/black/

Once I apply formatting though, and update my fork with latest changes from origin, the result is a whole lot of merge conflicts.

Therefore I kindly suggest that the repo contributors choose and apply a code formatter. Perhaps only in smaller/ shorter files as a start.

[huchenlei]

We can probably add a git presubmit hook to format code.