Add datetime object as valid StateType

[frenck]

Breaking change

When using datetime and date device classes in sensors, the returned native value must (respectively) be a datetime or date Python object.

Returning an iso formatted date(time) string in these cases is now deprecated and will write a deprecation warning in the logs. This fallback/backward compatibility will be removed in Home Assistant 2022.2.

Proposed change

This PR makes a datetime object a valid native state return type for sensors.

We currently accept: None, str, int and float
This PR adds datetime and date to it.

It is converted as an ISO 8601 string. This prevents issues like #52666.

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 #
• This PR is related to issue:
• Link to documentation pull request:

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

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  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.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

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To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[jjlawren]

Looks good!

[MartinHjelmare]

We should probably run this by Paulus since he had some reservations for Joakim's PR.

[frenck]

Interesting find in Z-Wave JS

https://github.com/home-assistant/core/blob/dev/tests/components/zwave_js/fixtures/climate_heatit_z_trm3_no_value_state.json#L1084-L1103

FAILED tests/components/zwave_js/test_climate.py::test_thermostat_heatit_z_trm3_no_value[pyloop] - ValueError: Invalid date/datetime: sensor.floor_thermostat_time_3 provide state '3.2', while it has device class 'timestamp'

Seems like one of the fixtures exposes a bug in the sensors device class handling?

[MartinHjelmare]

Looks good!