fix alertmanager genarate endsAt fields

[eveningcafe]

Description

There are no func nowdatetime in module datetime.

Reproduce:

from datetime import datetime, timedelta, timezone
datetime.nowdatetime.now(timezone.utc)

Unit-test miss checking that because you mock on fixed date :

Checklist

• [x] I have reviewed the contributing guidelines.
• [x] I have included unit tests for my changes or additions.
• [x] I have successfully run make test-docker with my changes.
• [x] I have manually tested all relevant modes of the change in this PR.
• [x] I have updated the documentation.
• [x] I have updated the changelog.

Questions or Comments