older readme_renderer doesn't work with docutils 0.13 (#3300)

diff --git a/setup.py b/setup.py
index b5c05df..c5a99be 100644
--- a/setup.py
+++ b/setup.py
@@ -313,7 +313,7 @@
         "docstest": [
             "doc8",
             "pyenchant",
-            "readme_renderer",
+            "readme_renderer >= 16.0",
             "sphinx",
             "sphinx_rtd_theme",
             "sphinxcontrib-spelling",