DOC: Don't import doctest because we're not using it

AFAICS we're not using it, so don't need to import it in conf.py

Author: timhoffm

doc/conf.py

@@ -57,7 +57,6 @@
 extensions = [
     'sphinx.ext.autodoc',
     'sphinx.ext.autosummary',
-    'sphinx.ext.doctest',
     'sphinx.ext.inheritance_diagram',
     'sphinx.ext.intersphinx',
     'sphinx.ext.ifconfig',