Import docutils types from new locations

This attempts to import ErrorString and SafeString from their new
locations in docutils, and falls back to their previous locations if the
import fails.

Fixes #8

sphinx_mdinclude/sphinx.py

@@ -6,10 +6,14 @@
 import os.path
 
 from docutils import io, nodes, statemachine, utils
-from docutils.core import ErrorString
 from docutils.parsers import rst
 from docutils.parsers.rst import directives as rst_directives
-from docutils.utils import SafeString
+
+try:  # new
+    from docutils.utils.error_reporting import ErrorString, SafeString
+except ImportError:  # old
+    from docutils.core import ErrorString
+    from docutils.utils import SafeString
 
 from . import RestMarkdown