[RLlib] Pin Gym to 0.19 (#19170)

Gym appears to have cut a release, 0.21.
It isn't clear what changes were made
between 0.19/0.20 and 0.21, as there is
no change log available for the 0.21 release,
so for now we'll pin gym to 0.19 until we
can fully understand the breaking changes
in gym 0.21. I suspect some things have
just been removed from the regular gym installation
that rllib has previously relied on. Will address
later.

ci/travis/install-dependencies.sh

@@ -408,7 +408,7 @@ install_dependencies() {
 
   # RLlib testing with TF 1.x.
   if [ "${RLLIB_TESTING-}" = 1 ] && { [ -n "${TF_VERSION-}" ] || [ -n "${TFP_VERSION-}" ]; }; then
-    pip install --upgrade tensorflow-probability=="${TFP_VERSION}" tensorflow=="${TF_VERSION}" gym
+    pip install --upgrade tensorflow-probability=="${TFP_VERSION}" tensorflow=="${TF_VERSION}" gym==0.19
   fi
 
   # Additional Tune dependency for Horovod.

python/requirements.txt

@@ -27,7 +27,7 @@ requests
 ## setup.py extras
 dm_tree
 flask
-gym
+gym==0.19
 lz4
 scikit-image
 opencv-python-headless==4.3.0.36