Pin pip to < 20.3 as requirements.txt is broken for 20.3.1 - temporar…

…y fix (#558)

* WIP: Trigger rebuild to reproduce error in bug#557

* Add tox/pip pinning to revert to old pip resolver

The 20.3.1 resolver (being more strict) doesn't accept the current
requirements.txt.  This patch reverts to an older pip so that work can
continue and the resolver issue will be resolved independently.

* Remove TODO line

tox.ini

@@ -7,6 +7,18 @@ sitepackages = false
 # NOTE(beisner): the 'py3' env is useful to "just give me whatever py3 is here."
 # NOTE(beisner): the 'py3x' envs are useful to use a distinct interpreter version (will fail if not found)
 ignore_basepython_conflict = true
+# 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}