DOC: drop unused extension doctest

Author: pavoljuhas

doc/manual/source/conf.py

@@ -31,8 +31,11 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.autodoc', 'sphinx.ext.doctest',
-              'sphinx.ext.coverage', 'sphinx.ext.napoleon']
+extensions = [
+    'sphinx.ext.autodoc',
+    'sphinx.ext.coverage',
+    'sphinx.ext.napoleon',
+]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']