Fix requirements issues.

This patch is the merge of 2 current fixes that need to be solved in the
same time. Indeed each issue lock our gates independently.

Dropping lower constraints testing

We facing errors related to the new pip resolver, this
topic was discussed on the ML and QA team proposed to
to test lower-constraints [1].

I propose to drop this test because the complexity and recurring pain needed
to maintain that now exceeds the benefits provided by this mechanismes.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.html

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



Co-authored-by: Daniel Bengtsson <dbengt@redhat.com>

Change-Id: Ia349eba36d8556d4d2775a83b9a344b2ebeed972