py/lexer: Raise NotImplError for unicode name escape, instead of assert.

py/lexer.c

@@ -29,6 +29,7 @@
 
 #include "py/mpstate.h"
 #include "py/lexer.h"
+#include "py/runtime.h"
 
 #define TAB_SIZE (8)
 
@@ -466,7 +467,7 @@ STATIC void mp_lexer_next_token_into(mp_lexer_t *lex, bool first_token) {
                                 // 3MB of text; even gzip-compressed and with minimal structure, it'll take
                                 // roughly half a meg of storage. This form of Unicode escape may be added
                                 // later on, but it's definitely not a priority right now. -- CJA 20140607
-                                assert(!"Unicode name escapes not supported");
+                                mp_not_implemented("unicode name escapes");
                                 break;
                             default:
                                 if (c >= '0' && c <= '7') {