Fix pickling error. (#2100) #9169

Job	Run time
benchmark (e10_drive)	4m 32s
base-tests (./cli)	4m 15s
benchmark (e11_platoon)	3m 24s
examples-rl (e10_drive)	10m 32s
base-tests (./envision)	5m 5s
examples-rl (e11_platoon)	4m 39s
base-tests (./smarts/core --nb-exec-timeout 65536 --ignore=./smarts/core/tests/test_notebook.py)	7m 39s
base-tests (./smarts/env --ignore=./smarts/env/tests/test_rllib_hiway_env.py)	6m 44s
base-tests (./smarts/env/tests/test_rllib_hiway_env.py)	5m 20s
base-tests (./smarts/sstudio)	5m 15s
base-tests (./smarts/ray)	5m 21s
base-tests (./examples/tests/test_examples.py)	8m 36s
	1h 11m 22s

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