add versionadded for getcheckinterval

Doc/lib/libsys.tex

@@ -200,6 +200,7 @@ It is always available.
 \begin{funcdesc}{getcheckinterval}{}
   Return the interpreter's ``check interval'';
   see \function{setcheckinterval()}.
+  \versionadded{2.3}
 \end{funcdesc}
 
 \begin{funcdesc}{getdefaultencoding}{}