Copy backport of isolated_modules from importlib_resources.

tests/compat/py312.py

@@ -0,0 +1,18 @@
+import contextlib
+
+from .py39 import import_helper
+
+
+@contextlib.contextmanager
+def isolated_modules():
+    """
+    Save modules on entry and cleanup on exit.
+    """
+    (saved,) = import_helper.modules_setup()
+    try:
+        yield
+    finally:
+        import_helper.modules_cleanup(saved)
+
+
+vars(import_helper).setdefault('isolated_modules', isolated_modules)

tests/compat/py39.py

@@ -4,3 +4,4 @@
 os_helper = try_import('os_helper') or from_test_support(
     'FS_NONASCII', 'skip_unless_symlink'
 )
+import_helper = try_import('import_helper') or from_test_support()