Add py38 to tox and travis

[clbarnes]

Can wait until after the windows tests go in.

[clbarnes]

Although maybe this has somehow prevented travis from detecting the config at all? It doesn't seem to be running. tox works locally though.

[aaronshifman]

Is travis active it doesn't seem to be checking any of the PRs?

[clbarnes]

Up, apparently https://www.traviscistatus.com/ 🤷‍♂️ I tried to activate it on my fork but travis couldn't find the repo.

[aaronshifman]

Sorry I meant is the repo linked to Travis - I don't have config access so I can't check

[psilentp]

@clbarnes I should have pinged you in #54 since you did all the work to get the tests running. I set up the CI with github actions since:

1. it doesn't require maintaining a separate account
2. maybe better windows support.

Mostly 1. since the last point is totally speculative on the current ownership. Happy to switch things over to travis - but I think we should probably stick with one CI pipeline.

[clbarnes]

Ah yes, I see now. I'm in favour of sticking to one CI pipeline if it lets us share config between OSs, which it seems to! I haven't used github actions before. I'll scrap this PR. If you haven't already, you could delete .travis.yml in your PR and then we can add 3.8 to that, probably best as a separate PR after it's been merged.