bpo-26219: Fix compiler warning in _PyCode_InitOpcache() (GH-13997)

Fix MSVC warning:

    objects\codeobject.c(285): warning C4244: '=':
    conversion from 'Py_ssize_t' to 'unsigned char',
    possible loss of data

Author: vstinner

Objects/codeobject.c

@@ -282,7 +282,7 @@ _PyCode_InitOpcache(PyCodeObject *co)
         co->co_opcache = NULL;
     }
 
-    co->co_opcache_size = opts;
+    co->co_opcache_size = (unsigned char)opts;
     return 0;
 }