Change output template filters timestamp_local and timestamp_utc to isoformat

[jbouwh]

Breaking change

The timestamp_local and timestamp_utc template functions/filters have changed their output format. The timestamp is now formatted to iso standards and contains timezone information.

Previously a timestamp from these would look like: 2016-07-21 16:39:04. Now it is formatted as: 2016-07-21T16:39:04+01:00.

This change may break templates that need a specific format, in those cases, you can use the timestamp_custom template filter instead.

Proposed change

The current template filters timestamp_local and timestamp_utc do not add timezone information. When used with MQTT to assign a sensor with device_class 'timestamp` timezone information must be available or an exception will be thrown.

This PR changes the output for the template filters timestamp_local and timestamp_utc to isoformat(), this ensures that current users of these filters have no timezone issues.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #MQTT Sensor warnings/errors with device_class timestamp - no usable template filters #60029
• This PR is related to issue:
• Link to documentation pull request: ISO format for timstamp_local and timestamp_utc home-assistant.io#20424

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

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  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
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[frenck]

Updated breaking change section a bit from my format, hope you don't mind.

[emontnemery]

Thanks, @jbouwh 👍