chore(deps): Remove gunicorn version upper bound (#276)

Author: HKWinterhalter

setup.py

@@ -54,7 +54,7 @@
         "flask>=1.0,<3.0",
         "click>=7.0,<9.0",
         "watchdog>=1.0.0",
-        "gunicorn>=19.2.0,<21.0; platform_system!='Windows'",
+        "gunicorn>=19.2.0; platform_system!='Windows'",
         "cloudevents>=1.2.0,<2.0.0",
     ],
     entry_points={