docs: Fix code-block background in old sphinxen

Apparently they set a background-color for these, when we just want
them to inherit the background-color from the highlight div.

[ci skip]

doc_src/python_docs_theme/static/pydoctheme.css

@@ -138,6 +138,8 @@ div.body a:hover {
 tt, code, pre {
     font-family: monospace, sans-serif;
     font-size: 96.5%;
+    /* Older sphinx versions set a color here, we need to unset it so we inherit from the outer div.highlight */
+    background-color: unset;
 }
 
 div.body tt, div.body code {