Replace aiocron

[jghibiki]

• Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Replaces the aiocron dependency with a simpler implementation that makes it easier to correctly get the time that a script is scheduled to run at.

• What is the current behavior? (You can also link to an open issue here)

The method for calculating the time a script will run at is off by ~20 minutes

• What is the new behavior (if this is a feature change)?

The time a script will run is calculated correctly.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No breaking changes, it has been tested against the latest code on master.

• Other information:

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