Support editable installs

[cdce8p]

Proposed change

Followup to #12770
While moving the project metadata from setup.cfg to pyproject.toml, I missed that setuptools doesn't yet support editable installs with only pyproject.toml. It requires a dummy setup.py or setup.cfg file. pypa/setuptools#2816

This isn't an issue for core since the setup.cfg wasn't empty and thus still exists.
I'll add a comment there too, however. Just to be safe.

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

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