py: Clean up includes.

Remove unnecessary includes. Add includes that improve portability.
diff --git a/py/objstr.c b/py/objstr.c
index 64ba6c5..9762b0e 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -1,6 +1,4 @@
-#include <stdlib.h>
-#include <stdint.h>
-#include <stdarg.h>
+#include <stdbool.h>
 #include <string.h>
 #include <assert.h>