Add sphinx back-compatablity import (#187)

myst_parser/myst_refs.py

@@ -12,12 +12,17 @@
 
 from sphinx import addnodes
 from sphinx.addnodes import pending_xref
-from sphinx.errors import NoUri
 from sphinx.locale import __
 from sphinx.transforms.post_transforms import ReferencesResolver
 from sphinx.util import docname_join, logging
 from sphinx.util.nodes import clean_astext, make_refnode
 
+try:
+    from sphinx.errors import NoUri
+except ImportError:
+    # sphinx < 2.1
+    from sphinx.environment import NoUri
+
 logger = logging.getLogger(__name__)