ZoneInfo.clear_cache can have Optional input (#8856)

Author: sobolevn

stdlib/zoneinfo/__init__.pyi

@@ -18,7 +18,7 @@ class ZoneInfo(tzinfo):
     @classmethod
     def from_file(cls: type[Self], __fobj: _IOBytes, key: str | None = ...) -> Self: ...
     @classmethod
-    def clear_cache(cls, *, only_keys: Iterable[str] = ...) -> None: ...
+    def clear_cache(cls, *, only_keys: Iterable[str] | None = ...) -> None: ...
 
 # Note: Both here and in clear_cache, the types allow the use of `str` where
 # a sequence of strings is required. This should be remedied if a solution