Minor cleanup of SensorEntity

[emontnemery]

Proposed change

Minor cleanup of SensorEntity:

• Remove seom backwards compatibility code from SensorEntity which is not needed when all integrations have been migrated.
  • This means the _attr_state is no longer supported for SensorEntity
• Remove dead code introduced by Add support for total and total_increasing sensor state classes #54523

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 is related to issue: Move temperature conversions to sensor base class (1/8) #48261

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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• 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.

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[emontnemery]

The failing tests will be fixed by #54623, this PR needs to be rebased once that's merged
The PR is rebased and the tests should now pass

[frenck]

We should also mark the state method final and re-type _attr_state: None = None.

[MartinHjelmare]

Does this have any breaking impact on custom integrations?

[ludeeus]

Does this have any breaking impact on custom integrations?

Potentially, I scanned 576 custom integrations, 3 was using _attr_state (1 did not inherit SensorEntity so unaffected), the other 2 got a PR to move them to _attr_native_value

[MartinHjelmare]

Looks good!