Switch to Python 3.6 for flake8 runs, re-enable E999 checks

.flake8

@@ -12,10 +12,9 @@
 # Nice-to-haves ignored for now
 #   152 E128 continuation line under-indented for visual indent
 #    43 E127 continuation line over-indented for visual indent
-#     6 E999 invalid syntax - FIXME: re-enable after flake8 is running from Python 3.6
 
 [flake8]
-ignore = F401, F811, E127, E128, E301, E302, E305, E501, E701, E704, E999, B303
+ignore = F401, F811, E127, E128, E301, E302, E305, E501, E701, E704, B303
 # We are checking with Python 3 but many of the stubs are Python 2 stubs.
 # A nice future improvement would be to provide separate .flake8
 # configurations for Python 2 and Python 3 files.