def create_enterprise_session(url, token=None):
    """
    Create a github3.py session for a GitHub Enterprise instance

    If token is not provided, will attempt to use the GITHUB_API_TOKEN
    environment variable if present.
    """

    gh_session = github3.enterprise_login(url=url, token=token)

    if gh_session is None:
        msg = 'Unable to connect to GitHub Enterprise (%s) with provided token.'
        raise RuntimeError(msg, url)

    return gh_session