py: Clean up includes.

Remove unnecessary includes. Add includes that improve portability.
diff --git a/py/lexer.c b/py/lexer.c
index 26fec12..58d54b6 100644
--- a/py/lexer.c
+++ b/py/lexer.c
@@ -1,9 +1,9 @@
 /* lexer.c -- simple tokeniser for Python implementation
  */
 
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
-#include <string.h>
 #include <assert.h>
 
 #include "misc.h"