license.bbclass: implement ast.NodeVisitor.visit_Constant

Since Python 3.8 visit_Num(), visit_Str() and so on are all deprecated
and replaced with visit_Constant.  We can't yet remove the deprecated
functions until we require 3.8, but we can implement visit_Constant to
silence the deprecation warnings.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>