Return new Command object that will drop a specified
        environment variable if it is set.

            new_cmd = old_cmd.without_env("PYTHON_PATH")

def without_env(self, environment_variable):
        """
        Return new Command object that will drop a specified
        environment variable if it is set.

            new_cmd = old_cmd.without_env("PYTHON_PATH")
        """
        new_command = copy.deepcopy(self)
        new_command._env_drop.append(str(environment_variable))
        return new_command