Add type to Session begin and begin_nested (#108)

Author: isra17

sqlalchemy-stubs/orm/session.pyi

@@ -42,8 +42,8 @@ class Session(_SessionClassMethods):
     connection_callable: Any = ...
     @property
     def info(self): ...
-    def begin(self, subtransactions: bool = ..., nested: bool = ...): ...
-    def begin_nested(self): ...
+    def begin(self, subtransactions: bool = ..., nested: bool = ...) -> SessionTransaction: ...
+    def begin_nested(self) -> SessionTransaction: ...
     def rollback(self): ...
     def commit(self): ...
     def prepare(self): ...