fix pygame rendering dependency

setup.py

@@ -15,7 +15,7 @@ extras = {
     "box2d": ["box2d-py==2.3.5", "pyglet>=1.4.0"],
     "classic_control": ["pyglet>=1.4.0"],
     "mujoco": ["mujoco_py>=1.50, <2.0"],
-    "toy_text": ["scipy>=1.4.1"],
+    "toy_text": ["pygame==2.1.0", "scipy>=1.4.1"],
     "other": ["lz4>=3.1.0", "opencv-python>=3.0"],
 }