At times an external program needs to be called, which requires setting PYTHONPATH in os.environ to include
multiple local paths. A helper class transformers.test_utils.TestCasePlus comes to help:
thon
from transformers.testing_utils import TestCasePlus
class EnvExampleTest(TestCasePlus):
    def test_external_prog(self):
        env = self.get_env()
        # now call the external program, passing env to it