Adjust path_to_url et al. to produce the same results on Python 3.14+

See python/cpython#125974
and #13138 (comment)
and https://discuss.python.org/t/pathname2url-changes-in-python-3-14-breaking-pip-tests/97091

Author: hroncok

news/13423.bugfix.rst

@@ -0,0 +1 @@
+Fix remaining test failures in Python 3.14 by adjusting ``path_to_url`` and similar functions.

src/pip/_internal/models/link.py

@@ -6,6 +6,7 @@
 import os
 import posixpath
 import re
+import sys
 import urllib.parse
 from collections.abc import Mapping
 from dataclasses import dataclass
@@ -131,7 +132,11 @@ def _clean_file_url_path(part: str) -> str:
     # should not be quoted. On Linux where drive letters do not
     # exist, the colon should be quoted. We rely on urllib.request
     # to do the right thing here.
-    return urllib.request.pathname2url(urllib.request.url2pathname(part))
+    ret = urllib.request.pathname2url(urllib.request.url2pathname(part))
+    if sys.version_info >= (3, 14):
+        # https://discuss.python.org/t/pathname2url-changes-in-python-3-14-breaking-pip-tests/97091
+        ret = ret.removeprefix("//")
+    return ret
 
 
 # percent-encoded:                   /

src/pip/_internal/utils/urls.py

@@ -12,7 +12,7 @@ def path_to_url(path: str) -> str:
     quoted path parts.
     """
     path = os.path.normpath(os.path.abspath(path))
-    url = urllib.parse.urljoin("file:", urllib.request.pathname2url(path))
+    url = urllib.parse.urljoin("file://", urllib.request.pathname2url(path))
     return url
 
 

tests/unit/test_urls.py

@@ -11,7 +11,7 @@
 def test_path_to_url_unix() -> None:
     assert path_to_url("/tmp/file") == "file:///tmp/file"
     path = os.path.join(os.getcwd(), "file")
-    assert path_to_url("file") == "file://" + urllib.request.pathname2url(path)
+    assert path_to_url("file") == "file://" + path
 
 
 @pytest.mark.skipif("sys.platform != 'win32'")