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.

Related-Bug: #2002788
Change-Id: Ie56b7d8506868db45d116a1fd31db3a24361f149

.gitreview

@@ -2,3 +2,5 @@
 host=review.opendev.org
 port=29418
 project=openstack/charm-ceph-radosgw.git
+
+defaultbranch=stable/quincy.2

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