Remove Trio

Trio is a rather old cross-platform printf library which was bundled with
libxml2. It was needed for ancient pre-C99 systems without snprintf and
should be safe to remove these days.

python/libxml.c

@@ -29,11 +29,6 @@
 #include "libxml_wrap.h"
 #include "libxml2-py.h"
 
-#if defined(WITH_TRIO)
-#include "trio.h"
-#define vsnprintf trio_vsnprintf
-#endif
-
 #if PY_MAJOR_VERSION >= 3
 PyObject *PyInit_libxml2mod(void);