Freeze version of dm-control in requirements.txt files.

A new version of dm_control is about to come out, which will depend on MuJoCo 2.1.1 (instead of 2.1.0), and we don't want to break existing users of dm_control until they've upgraded their MuJoCo installation.

PiperOrigin-RevId: 417360341

Author: nimrod-gileadi

physics_planning_games/requirements.txt

@@ -1,5 +1,5 @@
 absl-py == 0.9.0
-dm-control
+dm-control == 0.0.403778684
 dm-env
 labmaze
 numpy == 1.19.1