use new style find python

[hmaarrfk]

See https://cmake.org/cmake/help/latest/module/FindPython.html for more info

and the results of
conda-forge/nifty-feedstock#19

[hmaarrfk]

You might also just want to delete: https://github.com/hmaarrfk/nifty/blob/patch-1/conda-recipe/build.sh.old

[hmaarrfk]

Hmm FindVigra and likely most other things from: https://github.com/hmaarrfk/nifty/tree/patch-1/cmake/modules should just be removed. sorry for that noise feel free to force push to revert anything here.

[constantinpape]

Thanks @hmaarrfk. It looks like the CI is outdated. I will fix this in a separate PR.