Require redis-py<3.0.0

Redis-py 3.0.0 contains a breaking bug:
redis/redis-py#1128

Author: Chronial

setup.py

@@ -23,7 +23,10 @@ def get_version():
     zip_safe=False,
     platforms='any',
     python_requires='>=3.6',
-    install_requires=['redis >= 2.10.0', 'click', 'croniter >= 0.3.23', 'pytz'],
+    install_requires=['redis >= 2.10.0 <= 3.0.0',
+                      'click',
+                      'croniter >= 0.3.23',
+                      'pytz'],
     entry_points={
         'console_scripts': [
             'redis_tasks = redis_tasks.cli:main',