fix: Correctly reschedule tasks when they return a new trigger

[silasary]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

When I originally implemented the return trigger functionality, I missed that there was a .reschedule() method. I also missed that without using it, passing DateTriggers wouldn't work, as the task would have already stopped itself.

Changes

• Changed Task.__call__ to use the helper method, rather than assigning self.trigger directly.

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code