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.
         """

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -914,23 +914,17 @@ class DummyWritablePath(WritablePath, DummyJoinablePath):
         self._directories[parent].add(name)
         return DummyWritablePathIO(self._files, path)
 
-    def mkdir(self, mode=0o777, parents=False, exist_ok=False):
+    def mkdir(self):
         path = str(self)
         parent = str(self.parent)
         if path in self._directories:
-            if exist_ok:
-                return
-            else:
             raise FileExistsError(errno.EEXIST, "File exists", path)
         try:
             if self.name:
                 self._directories[parent].add(self.name)
             self._directories[path] = set()
         except KeyError:
-            if not parents:
             raise FileNotFoundError(errno.ENOENT, "File not found", parent) from None
-            self.parent.mkdir(parents=True, exist_ok=True)
-            self.mkdir(mode, parents=False, exist_ok=exist_ok)
 
     def symlink_to(self, target, target_is_directory=False):
         raise NotImplementedError