Bump the gym dependency to 0.18.0.
Issue
@@ -3,7 +3,7 @@ deprecated>=1.2.12
fasteners==0.15
# Version should be kept in step with @com_github_grpc_grpc in WORKSPACE.
grpcio>=1.36.0
-gym>=0.17.2
+gym>=0.18.0
humanize>=2.6.0
networkx==2.5
numpy>=1.19.3
