lower constraints job broken on stable branches older than ussuri

[Goutham Pacha Ravi]

This is a tracking bug to follow up on the lower-constraints test failures.

pip 20.3 brings in a strict dependency resolver which is enabled by default. This causes our lower-constraints tests to fail, because the requirements.txt, test-requirements.txt and lower constraints files were out of date with reality.

Here's how one can reproduce the failures:

 rm -rf ~/foo
 python3 -m venv foo
 source ~/foo/bin/activate
 python3 --version
 ~/foo/bin/pip install -U pip
 ~/foo/bin/pip install -c ~/requirements/upper-constraints.txt \
     -c ~/manila/lower-constraints.txt \
     -r ~/manila/test-requirements.txt \
     -r /opt/stack/manila/requirements.txt

This issue was addressed on maintained branches with:

master: https://review.opendev.org/c/openstack/manila/+/766661
stable/victoria: https://review.opendev.org/c/openstack/manila/+/766670
stable/ussuri: https://review.opendev.org/c/openstack/manila/+/766674

Mailing List posts discussing the issue:
* http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019362.html
* http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019672.html
* http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019659.html

[Goutham Pacha Ravi (gouthamr)]

Fix proposed to stable/train: https://review.opendev.org/c/openstack/manila/+/770053

[Goutham Pacha Ravi (gouthamr)]

Additional comments: http://eavesdrop.openstack.org/meetings/manila/2021/manila.2021-01-14-15.00.log.html#l-161

[Goutham Pacha Ravi (gouthamr)]

Lower constraints jobs have been dropped from extended maintenance releases:

stable/stein: https://review.opendev.org/771739
stable/rocky: https://review.opendev.org/770704
stable/queens: https://review.opendev.org/770864

It's acknowledged that the advertised lower bounds of some direct and transitive dependencies for manila are incorrect in these really old branches. Since we no longer make releases out of these branches, we don't have a way to signal requirements files updates; it's expected that deployers may have worked around any issues since these releases are at least 2 years old at this point.