Six package is not used anymore

Change-Id: Ie72645229f23692d6bf87e58dd8ef212dd91189c

In zed cycle, OpenStack projects and oslo lib
has dropped the py3.6 and py3.7 support

- https://governance.openstack.org/tc/reference/runtimes/zed.html

I also decrepated oslo-log < 5.0.0, so we should bump oslo.log>=5.0.0.

Because of oslo.log>=5.0.0 depends on oslo.i18n>=3.20.0 as below, so we
also need to upgrade oslo.i18n.

The conflict is caused by:
    The user requested oslo.i18n>=3.15.3
    python-saharaclient 1.4.0 depends on oslo.i18n>=3.15.3
    castellan 0.16.0 depends on oslo.i18n>=3.15.3
    keystonemiddleware 4.17.0 depends on oslo.i18n!=3.15.2 and >=2.1.0
    oslo-config 6.8.0 depends on oslo.i18n>=3.15.3
    oslo-concurrency 3.26.0 depends on oslo.i18n>=3.15.3
    oslo-db 6.0.0 depends on oslo.i18n>=3.15.3
    oslo-log 5.0.0 depends on oslo.i18n>=3.20.0
    The user requested (constraint) oslo-i18n==3.15.3

The conflict is caused by:
    The user requested pbr!=2.1.0 and >=2.0.0
    bashate 0.5.1 depends on pbr>=1.6
    fixtures 3.0.0 depends on pbr>=0.11
    stestr 1.0.0 depends on pbr!=2.1.0 and >=2.0.0
    testresources 2.0.0 depends on pbr>=1.8
    testtools 2.4.0 depends on pbr>=0.11
    python-saharaclient 1.4.0 depends on pbr!=2.1.0 and >=2.0.0
    castellan 0.16.0 depends on pbr!=2.1.0 and >=2.0.0
    keystoneauth1 3.4.0 depends on pbr!=2.1.0 and >=2.0.0
    keystonemiddleware 4.17.0 depends on pbr!=2.1.0 and >=2.0.0
    oslo-concurrency 3.26.0 depends on pbr!=2.1.0 and >=2.0.0
    oslo-context 2.22.0 depends on pbr!=2.1.0 and >=2.0.0
    oslo-db 6.0.0 depends on pbr!=2.1.0 and >=2.0.0
    oslo-i18n 3.20.0 depends on pbr!=2.1.0 and >=2.0.0
    oslo-log 5.0.0 depends on pbr>=3.1.1
    The user requested (constraint) pbr==2.0.0

The conflict is caused by:
    The user requested oslo.serialization!=2.19.1 and >=2.18.0
    python-saharaclient 1.4.0 depends on oslo.serialization!=2.19.1 and >=2.18.0
    keystonemiddleware 4.17.0 depends on oslo.serialization!=2.19.1 and >=1.10.0
    oslo-log 5.0.0 depends on oslo.serialization>=2.25.0
    The user requested (constraint) oslo-serialization==2.18.0

Change-Id: Iebd0c795dcdd72dac1141cfe18d2dad5201147c1

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

Change-Id: I60f1f739f25755cd344fe45e76d225712fedc40f

Add file to the reno documentation build to show release notes for
stable/yoga.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/yoga.

Sem-Ver: feature
Change-Id: I46567b4f8c29710c42767fc96ac0176f5f8bc3b7

Add file to the reno documentation build to show release notes for
stable/xena.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/xena.

Sem-Ver: feature
Change-Id: Ifed0f0ac32b4d957e7f0f4a4a41014ea6ea8e815

Add file to the reno documentation build to show release notes for
stable/wallaby.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/wallaby.

Sem-Ver: feature
Change-Id: I5d670a37e27605737d1491d8ae0cf7e64c8851b7