fix generation of epub (sphinx bug)

conf.py

@@ -454,6 +454,11 @@ def copy_legacy_redirects(app, docname): # Sphinx expects two arguments
                 os.makedirs(os.path.dirname(target_path))
             with open(target_path, "w") as f:
                 f.write(html)
-
+def fix_epub_generation(app):
+    # Ensure there's an epub handler
+    if "html" in app.registry.translation_handlers
+        app.registry.translation_handlers["epub"] = app.registry.translation_handlers["html"]
+
 def setup(app):
     app.connect('build-finished', copy_legacy_redirects)
+    fix_epub_generation(app)