[2.1.x] Fixed env.note_versionchange() deprecation warning in Sphinx 1.8.

Backport of a0ef6a0e from master

docs/_ext/djangodocs.py

@@ -12,6 +12,7 @@ from sphinx import addnodes
 from sphinx.builders.html import StandaloneHTMLBuilder
 from sphinx.directives import CodeBlock
 from sphinx.domains.std import Cmdoption
+from sphinx.errors import ExtensionError
 from sphinx.util import logging
 from sphinx.util.console import bold
 from sphinx.writers.html import HTMLTranslator
@@ -96,7 +97,11 @@ class VersionDirective(Directive):
         node['type'] = self.name
         if self.content:
             self.state.nested_parse(self.content, self.content_offset, node)
-        env.note_versionchange(node['type'], node['version'], node, self.lineno)
+        try:
+            env.get_domain('changeset').note_changeset(node)
+        except ExtensionError:
+            # Sphinx < 1.8: Domain 'changeset' is not registered
+            env.note_versionchange(node['type'], node['version'], node, self.lineno)
         return ret