Add type stub for requests.packages.urllib3.util.connection.HAS_IPV6 (#1436)

This is defined at https://github.com/requests/requests/blob/v2.14.2/requests/packages/urllib3/util/connection.py#L130

Author: mxr

third_party/2and3/requests/packages/urllib3/util/connection.pyi

@@ -2,6 +2,7 @@ from typing import Any
 
 poll = ...  # type: Any
 select = ...  # type: Any
+HAS_IPV6 = ...  # type: bool
 
 def is_connection_dropped(conn): ...
 def create_connection(address, timeout=..., source_address=..., socket_options=...): ...