bpo-36020: Remove snprintf macro in pyerrors.h (GH-20889)

On Windows, #include "pyerrors.h" no longer defines "snprintf" and
"vsnprintf" macros.

PyOS_snprintf() and PyOS_vsnprintf() should be used to get portable
behavior.

Replace snprintf() calls with PyOS_snprintf() and replace vsnprintf()
calls with PyOS_vsnprintf().
diff --git a/Include/pyerrors.h b/Include/pyerrors.h
index 399bb7c..979a26b 100644
--- a/Include/pyerrors.h
+++ b/Include/pyerrors.h
@@ -4,6 +4,8 @@
 extern "C" {
 #endif
 
+#include <stdarg.h>               // va_list
+
 /* Error handling definitions */
 
 PyAPI_FUNC(void) PyErr_SetNone(PyObject *);
@@ -307,21 +309,6 @@
     const char *reason          /* UTF-8 encoded string */
     );
 
-/* These APIs aren't really part of the error implementation, but
-   often needed to format error messages; the native C lib APIs are
-   not available on all platforms, which is why we provide emulations
-   for those platforms in Python/mysnprintf.c,
-   WARNING:  The return value of snprintf varies across platforms; do
-   not rely on any particular behavior; eventually the C99 defn may
-   be reliable.
-*/
-#if defined(MS_WIN32) && !defined(HAVE_SNPRINTF)
-# define HAVE_SNPRINTF
-# define snprintf _snprintf
-# define vsnprintf _vsnprintf
-#endif
-
-#include <stdarg.h>
 PyAPI_FUNC(int) PyOS_snprintf(char *str, size_t size, const char  *format, ...)
                         Py_GCC_ATTRIBUTE((format(printf, 3, 4)));
 PyAPI_FUNC(int) PyOS_vsnprintf(char *str, size_t size, const char  *format, va_list va)