add upper bound for sphinx

closes #63

Author: Chilipp

setup.py

@@ -36,7 +36,7 @@ def readme():
       license="Apache-2.0",
       packages=find_packages(exclude=['docs', 'tests*', 'examples']),
       install_requires=[
-          'Sphinx>=2.2',
+          'Sphinx>=2.2,<5.0',
       ],
       setup_requires=pytest_runner,
       tests_require=['pytest'],