Fix test name in debug message

Lib/packaging/tests/test_command_build_ext.py

@@ -370,7 +370,7 @@ def test_suite():
     src = _get_source_filename()
     if not os.path.exists(src):
         if verbose:
-            print ('test_build_ext: Cannot find source code (test'
+            print('test_command_build_ext: Cannot find source code (test'
                   ' must run in python build dir)')
         return unittest.TestSuite()
     else: