Fix RandomNumberGenerator pickling (#2639)

* Fix RandomNumberGenerator pickling

* Add test for RNG pickling fix

* Fix Python 3.7 compatibility about tuple unpacking

See: https://bugs.python.org/issue32117

* Fix formatting issue

* Add test for space pickling

tests/utils/test_seeding.py

@@ -1,3 +1,5 @@
+import pickle
+
 from gym import error
 from gym.utils import seeding
 
@@ -16,3 +18,13 @@ def test_valid_seeds():
     for seed in [0, 1]:
         random, seed1 = seeding.np_random(seed)
         assert seed == seed1
+
+
+def test_rng_pickle():
+    rng, _ = seeding.np_random(seed=0)
+    pickled = pickle.dumps(rng)
+    rng2 = pickle.loads(pickled)
+    assert isinstance(
+        rng2, seeding.RandomNumberGenerator
+    ), "Unpickled object is not a RandomNumberGenerator"
+    assert rng.random() == rng2.random()