Pin SFS-Python version

[mgeier]

Currently, only sfs is required.

Since the API of SFS-Python is likely to change from time to time, it would be good to require a specific version to make sure all code examples actually work.

[hagenw]

Good point, I will adjust this before the DAGA. I just have to check if I want to go with sfs 0.4 or 0.5

[mgeier]

You could start by pinning sfs==0.4.* now, because without that, the build will break with the upcoming release of SFS 0.5.0.

At some later point, you could update the code examples and change the required version to sfs==0.5.*.

[hagenw]

I updated the API to the upcoming sfs and pinned the version to sfs>=0.5.0