Backport 2020.06 (#2211)

Author: ziyiwu9494

setup.py

@@ -35,7 +35,10 @@
 
 EXTRAS['mujoco'] = [
     'mujoco-py<=2.0.2.8,>=2.0',
-    'gym[all]' + GYM_VERSION,
+    # Currently gym is not compatible with mujoco 2.0 because of poor
+    # performance. So here we just install imageio to meet the dependency
+    # requirement of gym's mujoco extra.
+    'imageio',
 ]
 
 EXTRAS['dm_control'] = [