py: Clean up includes.

Remove unnecessary includes. Add includes that improve portability.
diff --git a/py/emitnative.c b/py/emitnative.c
index df531af..3ed415b 100644
--- a/py/emitnative.c
+++ b/py/emitnative.c
@@ -16,8 +16,7 @@
 
 // for x in l[0:8]: can be compiled into a native loop if l has pointer type
 
-#include <unistd.h>
-#include <stdlib.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>