GH-127381: pathlib ABCs: remove WritablePath.mkdir() arguments (#130611)

Remove the *mode*, *parents* and *exist_ok* arguments from
`WritablePath.mkdir()`. These arguments imply support for POSIX permissions
and checking for preexistence of the path or its parents, but subclasses of
`WritablePath` may not have these capabilities.

The public `Path.mkdir()` method retains these arguments.

Lib/pathlib/_abc.py

@@ -352,7 +352,7 @@ class WritablePath(JoinablePath):
         raise NotImplementedError
 
     @abstractmethod
-    def mkdir(self, mode=0o777, parents=False, exist_ok=False):
+    def mkdir(self):
         """
         Create a new directory at this given path.
         """