gh-100809: Fix handling of drive-relative paths in pathlib.Path.absolute() (GH-100812)

Resolving the drive independently uses the OS API, which ensures it starts from the current directory on that drive.

Lib/test/support/os_helper.py

@@ -4,6 +4,7 @@ import errno
 import os
 import re
 import stat
+import string
 import sys
 import time
 import unittest
@@ -716,3 +717,37 @@ class EnvironmentVarGuard(collections.abc.MutableMapping):
             else:
                 self._environ[k] = v
         os.environ = self._environ
+
+
+try:
+    import ctypes
+    kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)
+
+    ERROR_FILE_NOT_FOUND = 2
+    DDD_REMOVE_DEFINITION = 2
+    DDD_EXACT_MATCH_ON_REMOVE = 4
+    DDD_NO_BROADCAST_SYSTEM = 8
+except (ImportError, AttributeError):
+    def subst_drive(path):
+        raise unittest.SkipTest('ctypes or kernel32 is not available')
+else:
+    @contextlib.contextmanager
+    def subst_drive(path):
+        """Temporarily yield a substitute drive for a given path."""
+        for c in reversed(string.ascii_uppercase):
+            drive = f'{c}:'
+            if (not kernel32.QueryDosDeviceW(drive, None, 0) and
+                    ctypes.get_last_error() == ERROR_FILE_NOT_FOUND):
+                break
+        else:
+            raise unittest.SkipTest('no available logical drive')
+        if not kernel32.DefineDosDeviceW(
+                DDD_NO_BROADCAST_SYSTEM, drive, path):
+            raise ctypes.WinError(ctypes.get_last_error())
+        try:
+            yield drive
+        finally:
+            if not kernel32.DefineDosDeviceW(
+                    DDD_REMOVE_DEFINITION | DDD_EXACT_MATCH_ON_REMOVE,
+                    drive, path):
+                raise ctypes.WinError(ctypes.get_last_error())