Remove the explicit setting of a pygments theme

[pradyunsg]

This allows the underlying theme being used to provide the pygments theme instead which looks better (subjectively).

Description

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.
• If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[pradyunsg]

• Create a file in src/towncrier/newsfragments/.

I don't think this is a news worthy change, but let me know if you want one.

[adiroiban]

Thanks for the update.

I triggerd the build so can check the new version on Read the docs.

I don't think this is a news worthy change, but let me know if you want one.

If it's not news worthy, then we need a newsfragment/625.misc file.

This confirm that the release notes information was considred and it was decided not to add any explicit text.

As opposed of not having a newsfragment file and thinking that the author of the PR has forgot about the release notes :)

[adiroiban]

This is subjective ... but I think that we can merge it.

Thanks :)