py: Check explicitly for memory allocation failure in parser.

Previously, a failed malloc/realloc would throw an exception, which was
not caught.  I think it's better to keep the parser free from NLR
(exception throwing), hence this patch.

py/parse.h

@@ -67,6 +67,7 @@ typedef enum {
 } mp_parse_input_kind_t;
 
 typedef enum {
+    MP_PARSE_ERROR_MEMORY,
     MP_PARSE_ERROR_UNEXPECTED_INDENT,
     MP_PARSE_ERROR_UNMATCHED_UNINDENT,
     MP_PARSE_ERROR_INVALID_SYNTAX,