Remove python 2 support

[hamogu]

This includes:

• [x] search for all appearances of six and fix to python 3
• [ ] convert np.dot(a, b) to a @ b for code readability
• [x] search for IGNORE_OUTPUT with marks doctests that have different output on 2 and 3
• [x] search for PY2 or PY3 and delete special cases.
• [x] check all docs use py3 syntax
• [x] write requirements into installation instructions

This issue will at least stay open, until the astropy infrastructure has support for running doctests in rst files. It is more important to test those to make sure they are up do date then to increase the code readability somewhat by saving a few lines.

[hamogu]

One big hurdle is that mayavi, the default plotting package is not available on python3 yet. MARXS runs independent of that, but it is so much more useful it it's available.

[hamogu]

Introduction of new language features (f string, a @ b, super does not require arguments any longer) can happen gradually when work on a specific module is done. No need to edit everything right now since the old version are still valid. The same is true for type annotations. They can be added as needed instead of one big editing effort to get them all in at once.

[hamogu]

Other changes: standard dicts now preserve order, no need for ordered dict any longer...