GH-123599: Remove duplicate url2pathname() implementation (#127237)

Call `urllib.request.url2pathname()` from `pathlib.Path.from_uri()` rather
than re-implementing it. This paves the way for solving the main issue
(ignoring local authorities and rejecting non-local ones) in urllib, not
pathlib.

Author: barneygale

Lib/pathlib/__init__.py

@@ -1271,21 +1271,8 @@ def from_uri(cls, uri):
         """Return a new path from the given 'file' URI."""
         if not uri.startswith('file:'):
             raise ValueError(f"URI does not start with 'file:': {uri!r}")
-        path = uri[5:]
-        if path[:3] == '///':
-            # Remove empty authority
-            path = path[2:]
-        elif path[:12] == '//localhost/':
-            # Remove 'localhost' authority
-            path = path[11:]
-        if path[:3] == '///' or (path[:1] == '/' and path[2:3] in ':|'):
-            # Remove slash before DOS device/UNC path
-            path = path[1:]
-        if path[1:2] == '|':
-            # Replace bar with colon in DOS drive
-            path = path[:1] + ':' + path[2:]
-        from urllib.parse import unquote_to_bytes
-        path = cls(os.fsdecode(unquote_to_bytes(path)))
+        from urllib.request import url2pathname
+        path = cls(url2pathname(uri.removeprefix('file:')))
         if not path.is_absolute():
             raise ValueError(f"URI is not absolute: {uri!r}")
         return path