Add "seconds" as a valid unit_of_measurement

[Leapo]

Proposed change

Simply adds "seconds" as a valid option for unit_of_measurement within the uptime counter, in addition to the existing options for minutes, hours, and days.

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

Example entry for configuration.yaml:

# Example configuration.yaml
sensor:
  - platform: uptime
    name: Seconds Online
    unit_of_measurement: seconds

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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[homeassistant]

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Thanks!

[balloob]

Great first contribution 👍

It still won't update every second because the update interval is the deafult 15 seconds. But that is good as it would spam the recorder.

(this integration should really just use self.initial.isoformat() as the state and a TIMESTAMP device class, but I guess it's already in 🤷 )