Style fixes: wrap long lines, fix hanging indents, bump flake8

### Changelog:
* Wrap *most* long lines (`E501`) in *most* python files.
* Fix all bad hanging indents (`E127`). Enable relevant build check.
* Bump version of `flake8`. New version supports multithreading, so `make style` will be a little bit faster.

(Please review; I 'll squash PR afterwards.)
Nice.