Pin pip < 20.3

Change-Id: I27af334ab6e66646a0fbb181a1436cf399c9bef7

tox.ini

@@ -7,6 +7,18 @@ skipsdist = True
 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}
@@ -24,8 +36,8 @@ deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:py35]
 basepython = python3.5
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+# python3.5 is irrelevant on a focal+ charm.
+commands = /bin/true
 
 [testenv:py36]
 basepython = python3.6