py: Clean up includes.

Remove unnecessary includes. Add includes that improve portability.
diff --git a/py/vstr.c b/py/vstr.c
index c518fa7..032acc6 100644
--- a/py/vstr.c
+++ b/py/vstr.c
@@ -1,3 +1,4 @@
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>