Port to python3 and new lalsuite

Hi Greg, I've recently been playing around with pyFstat, and wanted to update/modernize various aspects a bit.

If you're happy to integrate such changes into your upstream repo, I'll send you a few merge-requests, otherwise I'm also ok to keep this stuff on my fork.

@ReinhardPrix I for one would be very happy to see your updates included! I was going to return to using this code soon too.

mentioned in merge request !20 (merged)

merged

mentioned in commit 5c7e89a5