Move flake8 as a pre-commit local target.

The goal here is to avoid conflicts between flake8 and hacking version each
2 days.

Inspired from nova's approach[1].

The flake8 version to install will be determined by hacking and
requirements[2] will stay aligned instead of relying on different versions.

[1] https://opendev.org/openstack/nova/src/branch/master/.pre-commit-config.yaml#L26-L35
[2] https://opendev.org/openstack/hacking/src/branch/master/requirements.txt#L1

Change-Id: I27da03673da0663c77948d9193e74fc0e906632e