Fix problem when exec'ing a string with a coding
diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index 00bb38a..284082d 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -632,7 +632,7 @@
 				"unknown encoding: %s", tok->enc);
 			return error_ret(tok);
 		}
-		str = PyString_AsString(utf8);
+		str = PyBytes_AsString(utf8);
 	}
 	assert(tok->decoding_buffer == NULL);
 	tok->decoding_buffer = utf8; /* CAUTION */