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.

Contain also the patch[2] to remove the tox environment and the patch[3] to
remove the lower-constraints.txt file.

https://review.opendev.org/c/openstack/oslo.utils/+/787867

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.html
[2] https://review.opendev.org/c/openstack/oslo.utils/+/772630
[3] https://review.opendev.org/c/openstack/oslo.utils/+/787867

Change-Id: Ia02154a1eb04434658a21adf82599ae55302e310
(cherry picked from commit 4cc3a55824)
(cherry picked from commit c4e3576c63)
(cherry picked from commit 125617dcb4)

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - check-requirements
       - lib-forward-testing-python3
-      - openstack-lower-constraints-jobs
       - openstack-python3-ussuri-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti

lower-constraints.txt

@@ -1,40 +0,0 @@
-appdirs==1.3.0
-Babel==2.3.4
-bandit==1.4.0
-coverage==4.0
-ddt==1.0.1
-debtcollector==1.2.0
-eventlet==0.18.2
-extras==1.0.0
-fixtures==3.0.0
-funcsigs==1.0.0
-gitdb==0.6.4
-GitPython==1.0.1
-iso8601==0.1.11
-keystoneauth1==3.4.0
-linecache2==1.0.0
-mccabe==0.2.1
-netaddr==0.7.18
-netifaces==0.10.4
-os-client-config==1.28.0
-oslo.config==5.2.0
-oslo.i18n==3.15.3
-oslotest==3.2.0
-pbr==2.0.0
-pyparsing==2.1.0
-python-mimeparse==1.6.0
-python-subunit==1.0.0
-pytz==2013.6
-PyYAML==3.12
-requests==2.14.2
-requestsexceptions==1.2.0
-rfc3986==0.3.1
-six==1.10.0
-smmap==0.9.0
-stestr==2.0.0
-stevedore==1.20.0
-testscenarios==0.4
-testtools==2.2.0
-traceback2==1.4.0
-unittest2==1.1.0
-wrapt==1.7.0

tox.ini

@@ -49,9 +49,3 @@ deps =
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt