fix pyright.TypeIssues in test_mujoco_custom_env.py (#599)

tests/envs/mujoco/test_mujoco_custom_env.py

@@ -111,12 +111,14 @@ def test_frame_skip(frame_skip):
 def test_xml_file():
     """Verify that the loading of a custom XML file works"""
     relative_path = "./tests/envs/mujoco/assets/walker2d_v5_uneven_feet.xml"
-    env = PointEnv(xml_file=relative_path)
-    assert env.unwrapped.data.qpos.size == 9
+    env = PointEnv(xml_file=relative_path).unwrapped
+    assert isinstance(env, MujocoEnv)
+    assert env.data.qpos.size == 9
 
     full_path = os.getcwd() + "/tests/envs/mujoco/assets/walker2d_v5_uneven_feet.xml"
-    env = PointEnv(xml_file=full_path)
-    assert env.unwrapped.data.qpos.size == 9
+    env = PointEnv(xml_file=full_path).unwrapped
+    assert isinstance(env, MujocoEnv)
+    assert env.data.qpos.size == 9
 
     # note can not test user home path (with '~') because github CI does not have a home folder