[ruby/stringio] Development of 3.1.0 started.

https://github.com/ruby/stringio/commit/a2f8ef1a6a

ext/stringio/stringio.c

@@ -13,7 +13,7 @@
 **********************************************************************/
 
 static const char *const
-STRINGIO_VERSION = "3.0.9";
+STRINGIO_VERSION = "3.1.0";
 
 #include "ruby.h"
 #include "ruby/io.h"