docs: Fix Sphinx 3.x warnings, and enable warnings-as-errors on build.

This enables warnings as errors and fixes all current errors, namely:

- reference to terms in the glossary must now be explicit (:term:)
- method overloads must not be declared as a separate method or must
  use :noindex:
- 2 cases where `` should have been used instead of `

docs/templates/replace.inc

@@ -4,6 +4,6 @@
 
 .. |see_cpython_module| replace::
 
-    *This module implements a subset of the corresponding* `CPython` *module,
+    *This module implements a subset of the corresponding* :term:`CPython` *module,
     as described below. For more information, refer to the original
     CPython documentation:*