Switch Pyup bot to nightlies

[jacobtomlinson]

Instead of hard pinning dependencies in order to trigger CI to test new releases we should soft pin and introduce nightly builds to test new dependencies.

The downside of this is that an unrelated PR could discover the breakage before the nightly runs. Which isn't a great experience for newbies.

The plus side is installing opsdroid would be more pleasant and it would reduce some maintenance burden of reviewing and merging PyUp bot PRs.

[Cadair]

We should write a bot to report scheduled CI fails to the chat so we don't miss them when they break, if only we knew of a good bot framework 🤔 🤣