PYTHON-1347 - Replace usage of Sphinx Directive

sphinx.util.compat.Directive is deprecated. Use
docutils.parsers.rst.Directive instead.

doc/changelog.rst

@@ -10,6 +10,7 @@ Version 3.5.1 fixes bugs reported since the release of 3.5.0:
 - :meth:`pymongo.command_cursor.CommandCursor.close` now closes
   the cursor synchronously instead of deferring to a background
   thread.
+- Fix documentation build warnings with Sphinx 1.6.x.
 
 Issues Resolved
 ...............

doc/mongo_extensions.py

@@ -15,8 +15,8 @@
 """MongoDB specific extensions to Sphinx."""
 
 from docutils import nodes
+from docutils.parsers import rst
 from sphinx import addnodes
-from sphinx.util.compat import Directive
 
 
 class mongodoc(nodes.Admonition, nodes.Element):
@@ -48,7 +48,7 @@ def depart_mongoref_node(self, node):
         self.body.append('\n')
 
 
-class MongodocDirective(Directive):
+class MongodocDirective(rst.Directive):
 
     has_content = True
     required_arguments = 0