xtensor 0.27 requires C++ 20

[KrisThielemans]

From the xtensor README:

Up to version 0.26.0, xtensor requires a C++ compiler supporting C++14. xtensor 0.26.x requires a C++ compiler supporting C++17. xtensor 0.27.x requires a C++ compiler supporting C++20.

Seems we need to cope with this similarly to #262, although I think it's really a xtensor issue

[KrisThielemans]

@naegelejd not sure if you can release a new yardl version now, but potentially you fix this one as well. I'd rather have a release soon though, as we can easily work-around this one.