Docs: fix list spacing

Workaround sphinx-rtd-theme regression
on spacing in "open" lists.

docs/_static/anymail-theme.css

@@ -53,3 +53,20 @@ table.sticky-left th:first-of-type[colspan] > p {
   border-radius: 50px;
   padding: 6px 12px;
 }
+
+/* Improve list item spacing in "open" lists.
+   https://github.com/readthedocs/sphinx_rtd_theme/issues/1555
+
+   Undoes this rule in non-.simple lists:
+   https://github.com/readthedocs/sphinx_rtd_theme/blob/2.0.0/src/sass/_theme_rst.sass#L174-L175
+*/
+.rst-content .section ol:not(.simple) > li > p:only-child,
+.rst-content .section ol:not(.simple) > li > p:only-child:last-child,
+.rst-content .section ul:not(.simple) > li > p:only-child,
+.rst-content .section ul:not(.simple) > li > p:only-child:last-child,
+.rst-content section ol:not(.simple) > li > p:only-child,
+.rst-content section ol:not(.simple) > li > p:only-child:last-child,
+.rst-content section ul:not(.simple) > li > p:only-child,
+.rst-content section ul:not(.simple) > li > p:only-child:last-child {
+  margin-bottom: 12px;
+}