gh-118761: Add helper to ensure that lazy imports are actually lazy (#132614)

This ensures that if we jump through some hoops to make sure something is imported
lazily, we don't regress on importing it.

I recently already accidentally made typing import warnings and annotationlib eagerly.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Lib/test/test_typing.py

@@ -6317,6 +6317,15 @@ class InternalsTests(BaseTestCase):
             typing._collect_parameters
         self.assertEqual(cm.filename, __file__)
 
+    def test_lazy_import(self):
+        import_helper.ensure_lazy_imports("typing", {
+            "warnings",
+            "inspect",
+            "re",
+            "contextlib",
+            # "annotationlib",  # TODO
+        })
+
 
 @lru_cache()
 def cached_func(x, y):