pin futures at 2.2.0 and explain

requirements.txt

@@ -1,2 +1,6 @@
-futures
 six >=1.6
+futures <=2.2.0
+# Futures is not required for Python 3, but it works up through 2.2.0 (after which it introduced breaking syntax).
+# This is left here to make sure install -r works with any runtime. When installing via setup.py, futures is omitted
+# for Python 3, in favor of the standard library implementation.
+# see PYTHON-393