Nullable Episode.audio_preview_url

[Kuwertzel]

pydantic throws a ValidationError when an episode with the audio_preview_url field set to none is returned by the Spotify API. That is because the Episode base class currently expects str instead of Optional[str] for this attribute.

The Spotify API documentation declares the field as a nullable string.

I successfully tested the change using the following minimal example.
I'm not sure whether or not it would be appropriate to add a check for this change, but I did not create one. It feels wrong to use hard-coded episode IDs in a check since the episodes could change or be taken down in the future...

The existent checks seem to pass just fine, though i've never worked with tox before and might have overlooked something.

import tekore as tk

conf = ('<client_id>', '<client_secret>', 'http://localhost:8080/')
token = tk.prompt_for_user_token(*conf, scope=tk.scope.every)

spotify = tk.Spotify(token)

# works
print(f'with audio_preview_url: {spotify.episode("2VLjHtc7GdVcc2cokEbuI9").name}')

# does not work
print(f'without audio_preview_url: {spotify.episode("1pcBjfGIcXhSuoiBiPazHl").name}')

Related issue: I did not create one

• Tests written with 100% coverage
• Documentation and changelog entry written
• All tox checks passed with an appropriately configured
  environment

PS: This is my first proper pull request, so let me know about anything I can improve :)

[felix-hilden]

Thanks! Especially for submitting the PR directly! The change looks good 👍 I'll try to release it this week along with the other open PR. The tests fail due to missing secrets.