Prevent pylint import errors on six.moves

`six.moves` is a dynamically-created namespace that doesn't actually
exist and therefore `pylint` can't statically analyze it.

By default, `pylint` is smart enough to realize that and ignore the
import errors.

However, because we vendor it, the location changes to
`kafka.vendor.six.moves` so `pylint` doesn't realize it should be
ignored.

So this explicitly ignores it.

`pylint` documentation of this feature:
http://pylint.pycqa.org/en/1.9/technical_reference/features.html?highlight=ignored-modules#id34

More background:
* pylint-dev/pylint#1640
* pylint-dev/pylint#223

pylint.rc

@@ -1,5 +1,6 @@
 [TYPECHECK]
 ignored-classes=SyncManager,_socketobject
+ignored-modules=kafka.vendor.six.moves
 generated-members=py.*
 
 [MESSAGES CONTROL]