Modify sys.modules in place rather than replacing the dictionary

The lazy imports feature added in Python 3.15 holds a reference to the
original sys.modules, causing issues when the test fixture replaces it
during the test run. The tests, run individually, pass, but in the
sequential run - fail. See:
https://src.fedoraproject.org/rpms/python-rpm-macros/pull-request/201#comment-333958

tests/test_import_all_modules.py

@@ -20,7 +20,8 @@ def preserve_sys_path():
 def preserve_sys_modules():
     original_sys_modules = dict(sys.modules)
     yield
-    sys.modules = original_sys_modules
+    sys.modules.clear()
+    sys.modules.update(original_sys_modules)
 
 
 @pytest.mark.parametrize(