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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 _default_headers(self):

 """Set the default header for a Twilio SendGrid v3 API call"""

        headers = {
            "Authorization": 'Bearer {}'.format(self.api_key),
            "User-agent": self.useragent,
            "Accept": 'application/json'
        }
        if self.impersonate_subuser:
            headers['On-Behalf-Of'] = self.impersonate_subuser

        return headers