serd fails to build with Sphinx 7.x in Fedora 39 (currently Rawhide).

The issue detected:
```
+ sphinx-build -b html docs
<snip>
Theme error:
An error happened in rendering the page ....
Reason: UndefinedError("'style' is undefined")
```

Sphinx 7 has removed the `style` key for HTML templates (see: https://github.com/sphinx-doc/sphinx/pull/11381).
This can indicate an issue with the Sphinx theme the package uses rather than the package itself.

It should be possible to fix this by replacing the usage of `style` with `styles[-1] in `theme/layout.html`:
-    <link rel="stylesheet" href="{ pathto('_static/' + style, 1) }" type="text/css" />
+    <link rel="stylesheet" href="{ pathto('_static/' + styles[-1], 1) }" type="text/css" />

Please note it may not be the best solution in your specific case.

For the logs from testing build attempts, see:
https://copr.fedorainfracloud.org/coprs/ksurma/doc-stack/package/serd/

You can test you package in mock running:

$ mock -r fedora-rawhide-x86_64 --addrepo=https://download.copr.fedorainfracloud.org/results/ksurma/doc-stack/fedora-rawhide-x86_64/ --no-clean your.src.rpm
$ mock -r fedora-rawhide-x86_64 --addrepo=https://download.copr.fedorainfracloud.org/results/ksurma/doc-stack/fedora-rawhide-x86_64/ shell

We'd like to include Sphinx 7 in Fedora 39.
Let us know here if you have any questions. Thank you!