Fixed CodeBlock deprecation warning on Sphinx 2.1+.

docs/_ext/djangodocs.py

@@ -10,7 +10,7 @@ from docutils.parsers.rst import Directive
 from docutils.statemachine import ViewList
 from sphinx import addnodes
 from sphinx.builders.html import StandaloneHTMLBuilder
-from sphinx.directives import CodeBlock
+from sphinx.directives.code import CodeBlock
 from sphinx.errors import SphinxError
 from sphinx.domains.std import Cmdoption
 from sphinx.errors import ExtensionError