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.

Additionally, accommodate the following changes by hand from
origin/master:
- add libffi-dev in bindep.txt
- update pinning of charm-tools
  (but not unpinning to be in-sync with origin/master)
- update pinning of tempest

Related-Bug: #2002788
Change-Id: I8f4c84b412ed83c4f1a1f79ac87ec8ee57074fe1

bindep.txt

@@ -2,3 +2,4 @@ libxml2-dev [platform:dpkg test]
 libxslt1-dev [platform:dpkg test]
 build-essential [platform:dpkg test]
 zlib1g-dev [platform:dpkg test]
+libffi-dev [platform:dpkg test]

test-requirements.txt

@@ -44,7 +44,8 @@ git+https://github.com/openstack-charmers/zaza.git@stable/yoga#egg=zaza
 git+https://github.com/openstack-charmers/zaza-openstack-tests.git@stable/yoga#egg=zaza.openstack
 
 # Needed for charm-glance:
-git+https://opendev.org/openstack/tempest.git#egg=tempest;python_version>='3.6'
+git+https://opendev.org/openstack/tempest.git#egg=tempest;python_version>='3.8'
+tempest<31.0.0;python_version<'3.8' and python_version>='3.6'
 tempest<24.0.0;python_version<'3.6'
 
 croniter            # needed for charm-rabbitmq-server unit tests

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
@@ -84,7 +85,7 @@ deps = -r{toxinidir}/requirements.txt
 [testenv:pep8]
 basepython = python3
 deps = flake8==3.9.2
-       charm-tools==2.8.3
+       charm-tools==2.8.4
 commands = flake8 {posargs} hooks unit_tests tests actions lib files
            charm-proof