Create an HTTP session.

    Parameters
    ----------
    access_token : str
        Mapbox access token string (optional).
    env : dict, optional
        A dict that subsitutes for os.environ.

    Returns
    -------
    requests.Session

def Session(access_token=None, env=None):
    """Create an HTTP session.

    Parameters
    ----------
    access_token : str
        Mapbox access token string (optional).
    env : dict, optional
        A dict that subsitutes for os.environ.

    Returns
    -------
    requests.Session
    """
    if env is None:
        env = os.environ.copy()
    access_token = (
        access_token or
        env.get('MapboxAccessToken') or
        env.get('MAPBOX_ACCESS_TOKEN'))
    session = requests.Session()
    session.params.update(access_token=access_token)
    session.headers.update({
        'User-Agent': 'mapbox-sdk-py/{0} {1}'.format(
            __version__, requests.utils.default_user_agent())})
    return session