Pin tox to < 4.0.0

Tox 4.0.0 was recently released and it has several breaking changes.
We pin to < 4.0.0 here. We are planning to move forward only on the
master charm branches.

Tox is also pinned to < 4.0.0 for stable branches in upstream
openstack-zuul-jobs as well as in zosci-config. However, the
requires= section in the charm's tox.ini file ends up installing
tox>4, wiping out the zuul-pinned tox<4 that was already installed
installed. This patch fixes that.

The openstacksdk version is pinned in test-requirements.txt to enabled
py36 unit tests to run successfully.

Related-Bug: #2002788
Change-Id: I1783a67c9c4dcc2c840c164698bdf6cec9235e14

test-requirements.txt

@@ -36,6 +36,10 @@ stevedore<1.31.0;python_version<'3.6'
 debtcollector<1.22.0;python_version<'3.6'
 oslo.utils<=3.41.0;python_version<'3.6'
 
+# New openstacksdk versions depend on platformdirs>=3 which does not support
+# python 3.6
+openstacksdk<1.6.0; python_version <= '3.6'
+
 coverage>=4.5.2
 pyudev              # for ceph-* charm unit tests (need to fix the ceph-* charm unit tests/mocking)
 git+https://github.com/openstack-charmers/zaza.git@stable/wallaby#egg=zaza

tox.ini

@@ -26,6 +26,7 @@ requires =
   pip < 20.3
   virtualenv < 20.0
   setuptools < 50.0.0
+  tox < 4.0.0
 
 # NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
 minversion = 3.18.0