isort flags

[keewis]

Because I've been hit by this elsewhere: isort has released a new version today that removes the -rc / --recursive flag (without deprecation, I think). Once conda-forge has been updated, we will start seeing a failing isort CI.

Not sure if we should pin isort for now or simply remove the flag once the builds are failing (or we remove the flag now and have a pretty much useless isort CI until the feedstock has been merged).

Update: isort relaxed the error into a warning, so this is not that urgent