Add sensor state_class property

[emontnemery]

Proposed change

Add sensor state_class property as proposed in home-assistant/architecture#557
Use state_class property in philips_hue integration
Also, some minor typing improvements for sensor, can be moved to a separate PR.

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: Document sensor state_class developers.home-assistant#922

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.

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To help with the load of incoming pull requests:

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[pvizeli]

👍 LGTM - I think the best future approved choice

[MartinHjelmare]

Looks good!

[MartinHjelmare]

We should update the developer docs.

[emontnemery]

Developer docs PR: home-assistant/developers.home-assistant#922

[balloob]

Let's write a blog post for the dev blog about this so more integration authors are aware. We should also update our quality scale requirements.