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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 _login(self, user, password, restrict_login=None):
 """ Backend login method for Bugzilla3 """

        payload = {'login': user, 'password': password}
        if restrict_login:
            payload['restrict_login'] = True

        return self._proxy.User.login(payload)