Run isort as pre-commit hook

[janfabry]

Run it before flake8, so any errors will get fixed automatically.

Fixes #2711

Should we also remove flake8-isort from the flake8 check? And remove the flake8-isort requirement?

It's not clear to me yet whether pre-commit runs and installs its hooks in the same environment you are using, or in a separate virtualenv, so I don't know if it is an issue someone already has an older isort installed.

[browniebroke]

Should we also remove flake8-isort from the flake8 check?

We could, yes. Don't see a problem to keep it around, but maybe it is?

And remove the flake8-isort requirement?

I think it's best to keep it around for now.

It's not clear to me yet whether pre-commit runs and installs its hooks in the same environment you are using, or in a separate virtualenv

It's installing each hook in a separate virtualenv.

[browniebroke]

Going to merge this, it's a good improvement. Feel free to send another one if you see some issues with having flake8-isort along with flake8 pre-commit hook.