Fix setup.py for gym v21 (#88)

Co-authored-by: Mark Towers <mark.m.towers@gmail.com>

setup.py

@@ -33,7 +33,7 @@ def get_version():
 header_count, long_description = get_description()
 
 extras = {
-    "gym-v21": ["gym>=0.21.0", "pyglet==1.5.11"],
+    "gym-v21": ["gym>=0.21.0,<0.26", "pyglet==1.5.11"],
     "gym-v26": ["gym>=0.26.2"],
     "atari": ["ale-py~=0.8.1"],
     # "imageio" should be "gymnasium[mujoco]>=0.26" but there are install conflicts
@@ -49,7 +49,9 @@ def get_version():
     "meltingpot": ["pettingzoo>=1.23"],
     "bsuite": ["bsuite>=0.3.5"],
 }
-extras["all"] = list({lib for libs in extras.values() for lib in libs})
+extras["all"] = [
+    lib for key, libs in extras.items() if key != "gym-v21" for lib in libs
+]
 extras["testing"] = [
     "pytest==7.1.3",
     "pillow>=9.3.0",