Add python 3.8-dev to travis and tox

[andrey-git]

Description:

Add python 3.8-dev to travis and tox

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

[balloob]

I am not a really big fan. This adds 21 minutes to each and every build of Home Assistant.

[andrey-git]

13:38 on second run.

[andrey-git]

Third run 9:48 on par with other python versions

[balloob]

So should we have allow failure true? Because I remember having this at some point for 3.7 and all tests started failing because they accidentally broke something.

[andrey-git]

I think we should try without, and add it if Travis breaks and there is no easy fix.

[balloob]

The problem that once it breaks, the open PRs won't have allow failure true on it. Also usually breaking changes are not introduced in a PR but something existing breaks

[balloob]

Now that we allow failures, we are adding 25% time to every build with no real added value of seeing if things are going to break.

I wish that we could run a 3.8-dev build once a day, not on every build. Especially when I am dong a lot of code reviews, waiting for builds to finish can be a limiting factor (I sometimes make minor edits myself to get a PR through).

[andrey-git]

But those run in parallel (on Travis) and since 3.8 is allowed to fail Travis will Mark the build green without waiting for it.

[balloob]

Travis will only assign X workers (depends on time/how busy it is) to Home Assistant. So if there is more work to be done, builds will wait for other HA builds.

[andrey-git]

Now it will run only on pushes to dev and not on each PR change.
WDYT?

(We could make it run on pushes to master to make it really rare)

[balloob]

Perfect.