Pin more-itertools to a version supporting Python 2.7
Also add a comment about why we're specifying versions
of tox deps for Python 2.7.
[tox]
envlist = py,flake8,docs
minversion = 2.4.0
+# Pinning more-itertools and specifying virtualenv minimum version are
+# workaround hacks for Python 2.7 and can be dropped when we drop support for
+# Python 2.7.
requires = virtualenv >= 16.4.0
+ more-itertools==4.2.0
[testenv]
extras = tests
