Run tests with the unreleased Twisted

[wRAR]

To be able to catch incompatibilities in advance we want to run our tests with the master version of Twisted, ideally as a separate periodical job not tied to PRs. Related to #5303 but with a more narrow scope.

[Gallaecio]

#5303 could serve as an inspiration. It includes the use of cron (the idea here would be for a job that runs on a schedule, not on every pull request).

However, here we only want the latest Twisted, no other dependency, at least for now; and the test for the latest non-final Python version can be moved to this job as well.

We also want to make sure maintainers get a notification, e.g. by email, if it fails.

[glyph]

Having an automated process for this would be ideal, but I should also note that we always provide a week's worth of feedback period after an announcement email for all our downstream dependencies. We sent out an announcement on 8/18 about 23.8.0:

https://mail.python.org/archives/list/twisted@python.org/thread/22N6IAGSSOLMAXWLYG3OPTMOQIJH542I/

Bug reports during development that we can mark as release blockers would be great, but if all you can do is check that email and do a little manual testing to make sure that we haven't regressed or hold the release to give you an opportunity to fix a bug on your end, we can probably do that.