Create a :obj:`SlackClient` with a token from an env var.

def _create_slack_with_env_var(env_var: EnvVar) -> SlackClient:
    """ Create a :obj:`SlackClient` with a token from an env var. """
    token = os.getenv(env_var)
    if token:
        return SlackClient(token=token)
    raise MissingToken(f"Could not acquire token from {env_var}")