Add type stub for sqlalchemy.orm.session.close_all_sessions (#147)

Related code in sqlalchemy repository:
https://github.com/sqlalchemy/sqlalchemy/blob/ceba13d4be5e73fed4522d6f66ab4c54f60fd983/lib/sqlalchemy/orm/session.py#L3340-L3355
https://github.com/sqlalchemy/sqlalchemy/blob/ceba13d4be5e73fed4522d6f66ab4c54f60fd983/lib/sqlalchemy/orm/__init__.py#L40

Author: tvuotila

sqlalchemy-stubs/orm/__init__.pyi

@@ -31,6 +31,7 @@ from .descriptor_props import (
 from .relationships import foreign as foreign, remote as remote
 from .session import (
     Session as Session,
+    close_all_sessions as close_all_sessions,
     object_session as object_session,
     sessionmaker as sessionmaker,
     make_transient as make_transient,

sqlalchemy-stubs/orm/session.pyi

@@ -109,3 +109,4 @@ class sessionmaker(_SessionClassMethods):
 def make_transient(instance) -> None: ...
 def make_transient_to_detached(instance) -> None: ...
 def object_session(instance): ...
+def close_all_sessions() -> None: ...