pythongh-12933: fix import error over nfs on Windows

On Windows, realpath uses `GetFinalPathNameByHandleW` to resolve paths. This returns paths starting with a `\\?\` prefix, which is usually not what we want.

This change attempts to remove the prefix for \\?\UNC\ paths. A similar thing was already done for drive paths such as \\?\C:\.

Author: ChrisDenton

Misc/ACKS

@@ -438,6 +438,7 @@ Erik Demaine
 Jeroen Demeyer
 Martin Dengler
 John Dennis
+Chris Denton
 L. Peter Deutsch
 Roger Dev
 Philippe Devalkeneer

Modules/getpath.c

@@ -540,11 +540,24 @@ getpath_realpath(PyObject *Py_UNUSED(self) , PyObject *args)
         PyErr_SetFromWindowsErr(err);
         result = NULL;
     } else if (len <= MAXPATHLEN) {
-        const wchar_t *p = resolved;
+        wchar_t *p = resolved;
         if (0 == wcsncmp(p, L"\\\\?\\", 4)) {
-            if (GetFileAttributesW(&p[4]) != INVALID_FILE_ATTRIBUTES) {
-                p += 4;
-                len -= 4;
+            if (0 == wcsncmp(&p[4], L"UNC\\", 4)) {
+                // A \\?\UNC\ path. Try converting to a \\ path.
+                p[6] = L'\\';
+                if (GetFileAttributesW(&p[6]) != INVALID_FILE_ATTRIBUTES) {
+                    p += 6;
+                    len -= 6;
+                } else {
+                    // Change back to a \\?\UNC\ path.
+                    p[6] = L'C';
+                }
+            } else {
+                // Maybe a drive path like \\?\C:\. Try stripping the prefix.
+                if (GetFileAttributesW(&p[4]) != INVALID_FILE_ATTRIBUTES) {
+                    p += 4;
+                    len -= 4;
+                }
             }
         }
         if (CompareStringOrdinal(path, (int)pathlen, p, len, TRUE) == CSTR_EQUAL) {