regrtest: Fix module.__path__

Issue #26538: libregrtest: Fix setup_tests() to keep module.__path__ type
(_NamespacePath), don't convert to a list.

Add _NamespacePath.__setitem__() method to importlib._bootstrap_external.

Lib/importlib/_bootstrap_external.py

@@ -971,6 +971,9 @@ class _NamespacePath:
     def __iter__(self):
         return iter(self._recalculate())
 
+    def __setitem__(self, index, path):
+        self._path[index] = path
+
     def __len__(self):
         return len(self._recalculate())