Use os.path.dirname() for getting the directory name of a path

[akx]

Proposed change

Very tiny change I noticed while grepping for splits in #81493 – the meaning of os.path.dirname(...) is more easy to understand than os.path.split(...)[0].

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)
• [ ] Deprecation (breaking change to happen in the future)
• [ ] Breaking change (fix/feature causing existing functionality to break)
• [x] Code quality improvements to existing code or addition of tests

Checklist

• [x] The code change is tested and works locally.
• [ ] Local tests pass. Your PR cannot be merged unless tests pass
• [x] There is no commented out code in this PR.
• [x] I have followed the development checklist
• [x] The code has been formatted using Black (black --fast homeassistant tests)
• [ ] Tests have been added to verify that the new code works.
  • Nothing new to test.

To help with the load of incoming pull requests:

• [x] I have reviewed two other open pull requests in this repository.