test_cext, test_cppext: enable /W4 warnings on Windows (#124253)

Add an explicit cast to (void*) and add Py_UNUSED() to fix some
warnings in extension.c.

Author: vstinner

Lib/test/test_cext/extension.c

@@ -37,7 +37,13 @@ static PyMethodDef _testcext_methods[] = {
 
 
 static int
-_testcext_exec(PyObject *module)
+_testcext_exec(
+#ifdef __STDC_VERSION__
+    PyObject *module
+#else
+    PyObject *Py_UNUSED(module)
+#endif
+    )
 {
 #ifdef __STDC_VERSION__
     if (PyModule_AddIntMacro(module, __STDC_VERSION__) < 0) {
@@ -53,7 +59,7 @@ _testcext_exec(PyObject *module)
 }
 
 static PyModuleDef_Slot _testcext_slots[] = {
-    {Py_mod_exec, _testcext_exec},
+    {Py_mod_exec, (void*)_testcext_exec},
     {0, NULL}
 };
 

Lib/test/test_cext/setup.py

@@ -31,6 +31,8 @@
 else:
     # MSVC compiler flags
     CFLAGS = [
+        # Display warnings level 1 to 4
+        '/W4',
         # Treat all compiler warnings as compiler errors
         '/WX',
     ]

Lib/test/test_cppext/setup.py

@@ -22,6 +22,8 @@
 else:
     # MSVC compiler flags
     CPPFLAGS = [
+        # Display warnings level 1 to 4
+        '/W4',
         # Treat all compiler warnings as compiler errors
         '/WX',
     ]