Infra: Increase spacing between complex list items

[mhsmith]

A "simple" list is one whose items all consist of a single paragraph, optionally followed by a nested list which is itself simple. Sphinx detects such lists and gives them the simple class, but does not make any other structural changes to the HTML output.

The PEP style sheet currently does nothing with this class. As a result, when a list contains a mixture of single-paragraph and multi-paragraph items, it's common for a paragraph to appear closer to paragraphs of a different item than it is to other paragraphs of the same item. This obscures the logical structure of the text.

This PR fixes that by adding spacing between the items of "complex" (i.e. non-simple) lists, equal to the spacing between regular paragraphs. For example, in PEP 595:

BEFORE

AFTER

By definition, a simple list cannot contain a complex list. But if a complex list contains a simple list, it is still rendered compactly. For example, in PEP 572:

BEFORE

AFTER

A multi-level simple list only has the simple class on its top-level element. The rendering of such lists has not changed. For example, in PEP 432 (BEFORE / AFTER):

---

📚 Documentation preview 📚: https://pep-previews--3662.org.readthedocs.build/

[hugovk]

Thanks, changes look good with Chrome on macOS and Android.

It might be worth upstreaming these (to Sphinx?) but I'm fine including them here.

[mhsmith]

There are already some similar rules upstream in Sphinx's default themes, but the PEP theme has its own independent style sheet which isn't based on any of them.