Upgrade `flakehell` in the pre-commit hook

Created by: wdphy16

It seems that flakehell is not actively maintained, and it is incompatible with flake8 4.x released in last October (see flakehell/flakehell#22). That issue is not resolved after a few months. If a new developer of NetKet runs pre-commit install-hooks, it will just fail.

We may use this fix, or change it to flakeheaven which seems more actively maintained.