Python 3.9 support (#2088)

* try changes in pr 1804

* fix classifiers

* typo

.travis.yml

@@ -8,11 +8,12 @@ env:
   - PY_VER=3.6.12
   - PY_VER=3.7.9
   - PY_VER=3.8.6
+  - PY_VER=3.9.0
 
 install: "" # so travis doesn't do pip install requirements.txt
 script:
   - docker build -f py.Dockerfile --build-arg MUJOCO_KEY=$MUJOCO_KEY --build-arg PYTHON_VER=$PY_VER -t gym-test .
-  - docker run gym-test 
+  - docker run gym-test
 
 deploy:
     provider: pypi

gym/wrappers/pixel_observation.py

@@ -1,6 +1,7 @@
 """An observation wrapper that augments observations by pixel values."""
 
 import collections
+from collections.abc import MutableMapping
 import copy
 
 import numpy as np
@@ -8,6 +9,7 @@ import numpy as np
 from gym import spaces
 from gym import ObservationWrapper
 
+
 STATE_KEY = 'state'
 
 
@@ -60,7 +62,7 @@ class PixelObservationWrapper(ObservationWrapper):
             self._observation_is_dict = False
             invalid_keys = set([STATE_KEY])
         elif isinstance(wrapped_observation_space,
-                        (spaces.Dict, collections.MutableMapping)):
+                        (spaces.Dict, MutableMapping)):
             self._observation_is_dict = True
             invalid_keys = set(wrapped_observation_space.spaces.keys())
         else:

setup.py

@@ -48,5 +48,6 @@ setup(name='gym',
           'Programming Language :: Python :: 3.6',
           'Programming Language :: Python :: 3.7',
           'Programming Language :: Python :: 3.8',
+          'Programming Language :: Python :: 3.9',
       ],
 )