Fix Pytest no longer including setuptools

This broke coverage.

pants.ini

@@ -395,6 +395,12 @@ interpreter_cache_dir: %(pants_bootstrapdir)s/python_cache/interpreters
 resolver_cache_dir: %(pants_bootstrapdir)s/python_cache/requirements
 
 
+[pytest]
+# TODO(#8651): We need this until we switch to implicit namespace packages so that pytest-cov
+#  understands our __init__ files. NB: this version matches 3rdparty/python/requirements.txt.
+pytest_plugins: +["setuptools==40.6.3"]
+
+
 [test.pytest]
 fast: false
 chroot: true