fix the build with tornado6

backport of commit 15cfa135 from upstream

Index: setup.py
--- setup.py.orig
+++ setup.py
@@ -52,7 +52,7 @@ setup(
         'Programming Language :: Python :: 3 :: Only',
     ],
     python_requires='>=3.5',
-    install_requires=['tornado>=4.2,<6'],
+    install_requires=['tornado>=4.2,<7'],
     extras_require={
         'spelling': ['apertium-streamparser'],
         'suggestions': ['requests'],