Several packages always auto-import certain submodules on import (#1752)

* Importing yaml implies yaml.resolver

* Importing six implies six.moves

* Importing requests implies requests.packages

third_party/2and3/requests/__init__.pyi

@@ -6,6 +6,7 @@ from . import api
 from . import sessions
 from . import status_codes
 from . import exceptions
+from . import packages
 import logging
 
 __title__ = ...  # type: Any

third_party/2and3/yaml/__init__.pyi

@@ -5,6 +5,7 @@ from yaml.events import *  # noqa: F403
 from yaml.nodes import *  # noqa: F403
 from yaml.loader import *  # noqa: F403
 from yaml.dumper import *  # noqa: F403
+from . import resolver  # Help mypy a bit; this is implied by loader and dumper
 # TODO: stubs for cyaml?
 # from cyaml import *