[python-dateutil] Add missing gettz cache control methods

Also use literal `None` default argument for `__call__`.

source: https://github.com/dateutil/dateutil/blame/2.9.0.post0/src/dateutil/tz/tz.py#L1475
docs: https://dateutil.readthedocs.io/en/stable/tz.html#dateutil.tz.gettz

Author: brianschubert

stubs/python-dateutil/dateutil/tz/tz.pyi

@@ -131,7 +131,9 @@ def resolve_imaginary(dt: _DateTimeT) -> _DateTimeT: ...
 # Singleton type defined locally in a function. Calls itself "GettzFunc".
 @type_check_only
 class _GetTZ:
-    def __call__(self, name: str | None = ...) -> tzinfo | None: ...
+    def __call__(self, name: str | None = None) -> tzinfo | None: ...
+    def set_cache_size(self, size: int) -> None: ...
+    def cache_clear(self) -> None: ...
     def nocache(self, name: str | None) -> tzinfo | None: ...
 
 gettz: _GetTZ