Fix BaseModelFormSet.save_m2m: accept self (typeddjango#970)

* Fix BaseModelFormSet.save_m2m: accept `self`

Before it would result in the following when calling it:

> Attribute function "save_m2m" with type "Callable[[], None]" does not accept self argument  [misc]

* fixup! Fix BaseModelFormSet.save_m2m: accept `self`

django-stubs/forms/models.pyi

@@ -150,7 +150,7 @@ class BaseModelFormSet(Generic[_M, _ModelFormT], BaseFormSet[_ModelFormT]):
     def save_existing(self, form: _ModelFormT, instance: _M, commit: bool = ...) -> _M: ...
     def delete_existing(self, obj: _M, commit: bool = ...) -> None: ...
     saved_forms: List[_ModelFormT] = ...
-    save_m2m: Callable[[], None] = ...
+    def save_m2m(self) -> None: ...
     def save(self, commit: bool = ...) -> List[_M]: ...
     def clean(self) -> None: ...
     def validate_unique(self) -> None: ...