Pin tox to < 4.0.0, mock to < 5.0.0

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.

Also pin mock to < 5.0.0 to avoid it erroring out on some broken tests,
fixes for those tests are available in master but we can't backport
those properly until tox is fixed in the stable branches.

Related-Bug: #2002788
Change-Id: I2a665c9d7a6616ca99a276c38a45e1b06f89a073

test-requirements.txt

@@ -16,7 +16,7 @@ requests>=2.18.4
 # Newer mock seems to have some syntax which is newer than python3.5 (e.g.
 # f'{something}'
 mock>=1.2,<4.0.0; python_version < '3.6'
-mock>=1.2; python_version >= '3.6'
+mock>=1.2,<5.0.0; python_version >= '3.6'
 
 stestr>=2.2.0
 

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