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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def login(username=None, password=None, token=None, url=None, two_factor_callback=None):
 """Construct and return an authenticated GitHub session. This will return a GitHubEnterprise session if a url is provided. :param str username: login name :param str password: password for the login :param str token: OAuth token :param str url: (optional), URL of a GitHub Enterprise instance :param func two_factor_callback: (optional), function you implement to provide the Two Factor Authentication code to GitHub when necessary :returns: :class:`GitHub <github3.github.GitHub>` """

    g = None

    if (username and password) or token:
        g = GitHubEnterprise(url) if url is not None else GitHub()
        g.login(username, password, token, two_factor_callback)

    return g