when compiling with c++17, need swig > 4

[fbergmann]

As @alessandrofelder found out, the python bindings won't compile with swig 3.x on c++17, as it uses the removed unexpected_handler.

[fbergmann]

I dont think there is anything for us to change here, users are free to specify whichever swig version, and c++ standard they want to use. If anything it could be added to documentation.