Stringify enums in selectors

[balloob]

Breaking change

Proposed change

Blueprint importing validates a blueprint config before importing. The problem was that selector validation would convert certain strings to enums, causing the YAML dump to fail.

This PR makes sure that selectors stringify the enums and added a test to ensure that all future enums are stringified too.

Fixes #71396

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:

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.

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• Documentation added/updated for www.home-assistant.io

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  Updated and included derived files by running: python3 -m script.hassfest.
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  Updated by running python3 -m script.gen_requirements_all.
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[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (blueprint) you are listed as a code owner for? Thanks!
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[balloob]

Verified tests locally.