Change python requirement to 3.8 because of zuko

[Baschdl]

zuko as dependency requires at least python 3.8. Changed pyproject.toml and added sentence to CONTRIBUTING.md.

sbi installed via pip install sbi also seems to need at least python 3.8 (in contrast to what we write in the README). Using 3.7, it fails with

Package hdf5 was not found in the pkg-config search path.
Perhaps you should add the directory containing `hdf5.pc'
to the PKG_CONFIG_PATH environment variable
No package 'hdf5' found
reading from setup.cfg...

when installing netcdf4. Do we have to add h5py as additional dependency for 3.7?

The interplay of different Python versions and dependencies may also be something for continuous integration.