fix(pathlib2): missing exist_ok argument (#5218)

See https://github.com/mcmtroffaes/pathlib2/blob/91eafa818c777ec8a76c04e952530d3b1ce8bec1/pathlib2/__init__.py#L1527

Author: henryiii

stubs/pathlib2/pathlib2.pyi

@@ -65,7 +65,7 @@ class Path(PurePath):
     def iterdir(self) -> Generator[Path, None, None]: ...
     def lchmod(self, mode: int) -> None: ...
     def lstat(self) -> os.stat_result: ...
-    def mkdir(self, mode: int = ..., parents: bool = ...) -> None: ...
+    def mkdir(self, mode: int = ..., parents: bool = ..., exist_ok: bool = ...) -> None: ...
     # Adapted from _io.open
     def open(
         self,