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.

Additional changes included in this patch:
- Cherry-pick from commit 24d557ffdc
  by hand.
- Add bindep.txt to install libffi-dev. This is not needed in the
  master branch, likely because cffi is no longer in
  test-requiremetns.txt.

Related-Bug: #2002788
Change-Id: I231f6c19a574bb3f337451c98d2f7a8c0bafd65c

bindep.txt

@@ -0,0 +1 @@
+libffi-dev [platform:dpkg]

tests/bundles/jammy-yoga.yaml

@@ -17,9 +17,6 @@ machines:
   '5':
   '6':
   '7':
-  '8':
-    series: bionic  # nagios not supported on focal+ yet
-
 
 applications:
 

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