pythongh-120593: Add -Wcast-qual flag to test_cext

Author: vstinner

Lib/test/test_cext/setup.py

@@ -19,11 +19,14 @@
         '-Werror',
     ]
     if not support.Py_GIL_DISABLED:
-        CFLAGS.append(
+        CFLAGS.extend((
             # gh-116869: The Python C API must be compatible with building
             # with the -Werror=declaration-after-statement compiler flag.
-            '-Werror=declaration-after-statement',
-        )
+            '-Wdeclaration-after-statement',
+
+            # gh-120593: Check the const qualifier
+            '-Wcast-qual',
+        ))
 else:
     # Don't pass any compiler flag to MSVC
     CFLAGS = []