Blacklist some pyenchant versions (#3799)

diff --git a/setup.py b/setup.py
index cc3b706..fcc9eda 100644
--- a/setup.py
+++ b/setup.py
@@ -293,7 +293,7 @@
         "test": test_requirements,
         "docstest": [
             "doc8",
-            "pyenchant",
+            "pyenchant != 1.6.9, != 1.6.10",
             "readme_renderer >= 16.0",
             "sphinx != 1.6.1, != 1.6.2, != 1.6.3",
             "sphinx_rtd_theme",