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.

Added libxml2-dev to the list of dependencies in src/layer.yaml, this
explicit dependency is needed only when deploying on Bionic.

Related-Bug: #2002788
Change-Id: I4a5a4f3cafebb986c64e54d2a3a0bd048adf0933

src/layer.yaml

@@ -3,4 +3,4 @@ repo: https://github.com/openstack-charmers/charm-keystone-saml-mellon
 options:
   basic:
     use_venv: True
-    packages: [ 'libffi-dev', 'libssl-dev', 'libxslt1-dev' ]
+    packages: [ 'libffi-dev', 'libssl-dev', 'libxslt1-dev', 'libxml2-dev' ]

tox.ini

@@ -23,6 +23,7 @@ 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