lower min pyyaml version (#4004)

ml-agents-envs/setup.py

         "numpy>=1.14.1,<2.0",
         "Pillow>=4.2.1",
         "protobuf>=3.6",
-        "pyyaml>=5.1.1",
+        "pyyaml>=3.1.0",
     ],
     python_requires=">=3.5",
     cmdclass={"verify": VerifyVersionCommand},

ml-agents/setup.py

         "numpy>=1.13.3,<2.0",
         "Pillow>=4.2.1",
         "protobuf>=3.6",
-        "pyyaml",
+        "pyyaml>=3.1.0",
         "tensorflow>=1.7,<3.0",
         'pypiwin32==223;platform_system=="Windows"',
         # We don't actually need six, but tensorflow does, and pip seems