Switch to PEP-604 syntax in python2 stubs (#5915)

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>

stdlib/@python2/ensurepip/__init__.pyi

@@ -1,8 +1,6 @@
-from typing import Optional
-
 def version() -> str: ...
 def bootstrap(
-    root: Optional[str] = ...,
+    root: str | None = ...,
     upgrade: bool = ...,
     user: bool = ...,
     altinstall: bool = ...,