gh-120593: Check -Wcast-qual flag in test_cext (#121081)

Check the usage of the 'const' qualifier in the Python C API in
test_cext.

Author: vstinner

Lib/test/test_cext/setup.py

@@ -17,6 +17,9 @@
         # The purpose of test_cext extension is to check that building a C
         # extension using the Python C API does not emit C compiler warnings.
         '-Werror',
+
+        # gh-120593: Check the 'const' qualifier
+        '-Wcast-qual',
     ]
     if not support.Py_GIL_DISABLED:
         CFLAGS.append(