WR340583 update Sphinx to version 3.2.1

This requires a change to layout.html to add a new attribute
LINK_SUFFIX (same as FILE_SUFFIX) as the new version of searchtools.js
requires both of these values to display search results correctly.

requirements.txt

-Sphinx==2.3.1
+Sphinx==3.1.2
 sphinxjp.themes.basicstrap==0.5.0

source/mahara/layout.html

@@ -107,6 +107,7 @@
             VERSION:     '{{ release|e }}',
             COLLAPSE_INDEX: false,
             FILE_SUFFIX: '{{ '' if no_search_suffix else file_suffix }}',
+            LINK_SUFFIX: '{{ '' if no_search_suffix else file_suffix }}',
             HAS_SOURCE:  {{ has_source|lower }}
   };
 </script>