[BEAM-2964] Exclude incompatible six release.

Upstream bugs being tracked at google/apitools#175 and benjaminp/six#210

Author: robertwb

sdks/python/setup.py

@@ -113,7 +113,9 @@ def get_version():
 REQUIRED_TEST_PACKAGES = [
     'pyhamcrest>=1.9,<2.0',
     # Six required by nose plugins management.
-    'six>=1.9',
+    # Six 1.11.0 incompatible with apitools.
+    # TODO(BEAM-2964): Remove the upper bound.
+    'six>=1.9,<1.11',
     ]
 
 GCP_REQUIREMENTS = [