New rule: do not pass `deadline=<constant>` to `CancelScope()`, `move_on_at()`, or `fail_at()`

[Zac-HD]

These functions (docs) take a deadline= argument, which is defined relative to the current clock. That in turn means that there's never a good reason to pass a constant, but it's pretty easy to write nonsense without realizing:

with trio.fail_after(5): ...  # OK
with trio.fail_at(5): ...     # complete nonsense!

...but happily, it's also the kind of thing which is very easy to lint for.

[Zac-HD]

I'm actually pretty sceptical of the *_at() functions in general - it seems like most uses of absolute deadlines are mistakes, either more naturally expressed as relative deadlines or sometimes plain bugs where people have uses unix timestamps. But I'm happy to stick to linting the unambiguous cases for now.