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())
 

Lib/test/libregrtest/setup.py

@@ -41,8 +41,8 @@ def setup_tests(ns):
     # the packages to prevent later imports to fail when the CWD is different.
     for module in sys.modules.values():
         if hasattr(module, '__path__'):
-            module.__path__ = [os.path.abspath(path)
-                               for path in module.__path__]
+            for index, path in enumerate(module.__path__):
+                module.__path__[index] = os.path.abspath(path)
         if hasattr(module, '__file__'):
             module.__file__ = os.path.abspath(module.__file__)