Compatibility with python >= 3.14

[fkil]

Starting with python 3.14 the deprecated classes ast.Str and ast.Num have been removed. They are currently in use in miasm/core/sembuilder.py

Switching to using ast.Constant will, however, likely break compatibility with python versions lower than 3.8, which started representing all constants as ast.Constant.

I'm willing to provide a PR, but I'm not sure what the goal regarding compatibility is for newer miasm versions.