Add missing "gate" cover device_class

[pdecat]

Proposed change

This PR adds the missing "gate" device_class for covers:

See https://github.com/home-assistant/core/blob/0.118.1/homeassistant/components/cover/__init__.py#L54-L65
and home-assistant/core@0148708

Type of change

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

Example configuration

In customize.yaml:

cover.mygate:
  device_class: gate

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• 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