Pin pytest-xdist to < 1.28.0 in tests-requirements.txt.
@@ -6,7 +6,8 @@ pytest-cov
# Prevent installing 7.0 which has install_requires "pytest >= 3.10".
pytest-rerunfailures<7.0
pytest-timeout
-pytest-xdist
+# Prevent installing 1.28.0 which has install_requires "pytest >= 4.4.0".
+pytest-xdist<1.28.0
pyyaml
setuptools>=39.2.0 # Needed for `setuptools.wheel.Wheel` support.
scripttest
