Load a configuration from an environment variable pointing to
        a configuration file.

        :param variable_name: name of the environment variable
        :return: bool. ``True`` if able to load config, ``False`` otherwise.

def from_envvar(self, variable_name):
        """Load a configuration from an environment variable pointing to
        a configuration file.

        :param variable_name: name of the environment variable
        :return: bool. ``True`` if able to load config, ``False`` otherwise.
        """
        config_file = os.environ.get(variable_name)
        if not config_file:
            raise RuntimeError(
                "The environment variable %r is not set and "
                "thus configuration could not be loaded." % variable_name
            )
        return self.from_pyfile(config_file)