markdown: Make order of extensions deterministic

It makes the test from the previous commit pass reproducibly, and
not fail from time to time.

Refs retext-project/retext#98.

markups/markdown.py

@@ -215,7 +215,7 @@ def _apply_extensions(
                 extension_names.add(canonical_name)
                 extension_configs[canonical_name] = config
         self.md = self.markdown.Markdown(
-            extensions=list(extension_names),
+            extensions=sorted(extension_names),
             extension_configs=extension_configs,
             output_format="html5",
         )