setup.py: exclude Sphinx v3.4.2 which breaks docs

* The Read the Docs build with Sphinx v3.4.2 fails.
* See sphinx-doc/sphinx#8655 (already closed).

setup.py

@@ -45,7 +45,7 @@
 extras_require = {
     'all': [],
     'docs': [
-        'Sphinx>=1.8.5',
+        'Sphinx>=1.8.5,!=3.4.2',
         'sphinx-click>=2.1.0',
     ],
     'tests': tests_require,