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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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