docs: add space before attribute lists

Class/Module members/attributes were too close together in production
docs. Finally got motivated to fix it with a silly little CSS trick.

Skips over lists that are first-of-type to avoid adding too much space
between section headings and attribute lists with no lede text.

docs/source/_static/custom.css

@@ -0,0 +1,3 @@
+.section dl.py:not(:first-of-type) {
+    margin-top: 1.5em;
+}