Note for rb_deprecate_constant [ci skip]

Author: nobu

ext/etc/extconf.rb

@@ -47,6 +47,7 @@
   %x[#{RbConfig.ruby} #{srcdir}/mkconstants.rb -o #{srcdir}/constdefs.h]
 end
 
+# TODO: remove when dropping 2.7 support, as exported since 3.0
 have_func('rb_deprecate_constant(Qnil, "None")')
 
 $distcleanfiles << "constdefs.h"