Pin pytest-rerunfailures to < 7.0 in tests-requirements.txt.
@@ -3,7 +3,8 @@ mock
pretend
pytest==3.8.2
pytest-cov
-pytest-rerunfailures
+# Prevent installing 7.0 which has install_requires "pytest >= 3.10".
+pytest-rerunfailures<7.0
pytest-timeout
pytest-xdist
pyyaml
