[draft] Drop Python 3.7 and allow tests to work in isolation

[graingert]

[graingert]

for the last remaining tests it looks like there's a ThreadPoolExecutor running and not being shutdown. I'm not sure if this is a bug in distributed or dask-jobqueue right now

[guillaumeeb]

Thanks @graingert for all the contribution here. I think I'll need some time to digest all this.

I'm a bit confused by the modification for the running loop. Outside of test environment, the current code of dask-jobqueue seems to be working well with recent versions of distributed. And I didn't encounter these errors in my tests, how would you explain it ?