About specifying time_str in "at" scheduler

Hi, I appreciate for this project.
This is my first time to feedback.

This is relevant to #286

As in Doctsring, the `at` scheduler allows you to specify multiple execution times.
My idea is to unify the format and enforce the specification.
and actually, there is a bug in the behavior of the hourly scheduler.

Specifically, the execution unit (`Job.unit`) is
- `days` -> `HH:MM:SS`
- `hours` -> `MM:SS`
- Now, either `HH:SS` or `:SS` is allowed
- `minutes` -> `SS`

https://github.com/dbader/schedule/blob/41f88b33b8e20c8b87c0148d15f654ccea01440c/schedule/__init__.py#L381-L392

However, we know that the above changes will break backward compatibility.
I would be grateful if you could get feedback on this idea

Start with schedule/__init__.py at lines 381-392 and compare the proposal with issue #286. Clarify the accepted time_str formats for each Job.unit, the hourly scheduler behavior, and the backward-compatibility decision; the issue is complete only once those requirements have maintainer agreement.

Written by the indexing model from the issue text.