Sync pertinent tox.ini settings from release-tools

The new PIP resolver wreaks havoc for this repository.

Long term we should split unit test requirements into a separate
file and whip unit tests into shape wrt. mocking out everything as
opposed to relying on having random modules installed in the test
environment.

tox.ini

@@ -1,6 +1,22 @@
 [tox]
 envlist = pep8, py3
 skipsdist = True
+# NOTE: Avoid build/test env pollution by not enabling sitepackages.
+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
+# NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
+minversion = 3.2.0
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}