Add type stubs for new load strategies (#106)

Fixes #83

Also add in the other new load strategies. They are ordered in the way they are declared in the main sqlalchemy repo.

Author: ckarnell

sqlalchemy-stubs/orm/__init__.pyi

@@ -76,10 +76,13 @@ lazyload: Any = ...
 lazyload_all: Any = ...
 subqueryload: Any = ...
 subqueryload_all: Any = ...
+selectinload: Any = ...
+selectinload_all: Any = ...
 immediateload: Any = ...
 noload: Any = ...
 raiseload: Any = ...
 defaultload: Any = ...
+selectin_polymorphic: Any = ...
 
 def eagerload(*args, **kwargs): ...
 def eagerload_all(*args, **kwargs): ...