Release 0.8

[mrbean-bremen]

Collecting some ideas for the next version, mainly by going over the original issues:

• Standardize on a single marker name: "order" - I put this first, as it consolidates the existing markers, and it makes sense to do this before this fork is really used (if it ever will be...) -> done in release 0.7.1, together with renaming the repo
• Check/fix open bugs - go over the open bugs, check if they still exist, and fix them -> nothing to fix here for the current version, added enhancement issue for scope
• Ensure compatibility with xdist - there is a PR with respective tests, but I couldn't get them to work at first try
• Consolidate documentation - go over the documentation and make it consistent (currently a bit of a mix from some PRs) automate creation of ghpages, maybe make the schema nicer,
• Clean up copyright stuff - maybe just leave the original author's copyright in, but I'm not sure what I have to adapt / change for this fork, may need some help here -> clarified inheritance, left copyright in license
• Add support for Python 3.9, last version for Python 2.7?
• Sparse ordinal behavior - added in 0.8.1