Update CI and docs for new python versions

[clbarnes]

Python 3.10 is out, and because GHA > travis, we can use it in the build straight away rather than waiting 6+ months... We should also start testing on ubuntu 20.04 (vagrant already uses it so shouldn't be a problem).

We should also drop 3.6. The dependency molesq depends on recent numpy (maybe molesq doesn't need it, but it's a pain to find the MSnumpyV). Numpy (and most of the numerical stack) stopped supporting 3.6 in June last year; 3.6's proper EOL is at the end of the year anyway. This may necessitate a change in docs/ other scripts.

• Drop 3.6
  • [x] CI
  • [x] docs
  • [x] check docker, vagrant etc.
• Add 3.10
  • [x] CI
  • [x] Bump core numpy dep to match molesq's minimum
• [ ] Test on ubuntu 20.04
• [ ] Run lints etc. on py3.9

Once 3.6 is dropped, we will need to continue supporting 3.7 at least until pypy supports 3.8 (in the works but no predicted release date).

[clbarnes]

As of pypy 7.3.6, python 3.8 is supported. https://www.pypy.org/posts/2021/10/pypy-v736-release.html