Downgrade f-strings to good old format strings
Switch back to ordinary format strings instead of f-strings introduced in fc6e304a, i.e. make NIFTy work again on python < 3.6.
Switch back to ordinary format strings instead of f-strings introduced in fc6e304a, i.e. make NIFTy work again on python < 3.6.
This will create a new commit in order to revert the existing changes.