Sync in the tox/pip changes for blessings

The need to sync in the tox/pip changes for setuptools < 58 that is used
in other charms. Reactive charms depend on charm-tools, which depends on
blessings<=1.6 which uses 2to3 that is deprecated and removed in
setuptools 58.0.

Change-Id: I73b466e54335e4d75abc82e686cce6c2616b114f

pip.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+#
+# This file is managed centrally by release-tools and should not be modified
+# within individual charm repos.  See the 'global' dir contents for available
+# choices of tox.ini for OpenStack Charms:
+#     https://github.com/openstack-charmers/release-tools
+#
+# setuptools 58.0 dropped the support for use_2to3=true which is needed to
+# install blessings (an indirect dependency of charm-tools).
+#
+# More details on the beahvior of tox and virtualenv creation can be found at
+# https://github.com/tox-dev/tox/issues/448
+#
+# This script is wrapper to force the use of the pinned versions early in the
+# process when the virtualenv was created and upgraded before installing the
+# depedencies declared in the target.
+pip install 'pip<20.3' 'setuptools<50.0.0'
+pip "$@"

tox.ini

@@ -11,6 +11,21 @@ envlist = pep8,py3
 sitepackages = False
 # NOTE: Avoid false positives by not skipping missing interpreters.
 skip_missing_interpreters = False
+# NOTES:
+# * We avoid the new dependency resolver by pinning pip < 20.3, see
+#   https://github.com/pypa/pip/issues/9187
+# * Pinning dependencies requires tox >= 3.2.0, see
+#   https://tox.readthedocs.io/en/latest/config.html#conf-requires
+# * It is also necessary to pin virtualenv as a newer virtualenv would still
+#   lead to fetching the latest pip in the func* tox targets, see
+#   https://stackoverflow.com/a/38133283
+requires =
+  pip < 20.3
+  virtualenv < 20.0
+  setuptools<50.0.0
+
+# NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
+minversion = 3.18.0
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
@@ -21,7 +36,7 @@ setenv = VIRTUAL_ENV={envdir}
          JUJU_REPOSITORY={toxinidir}/build
 passenv = http_proxy https_proxy INTERFACE_PATH LAYER_PATH JUJU_REPOSITORY
 install_command =
-  pip install {opts} {packages}
+  {toxinidir}/pip.sh install {opts} {packages}
 deps =
     -r{toxinidir}/requirements.txt