Fetches the current version number of the Graph API being used.

def get_version(self):
        """Fetches the current version number of the Graph API being used."""
        args = {"access_token": self.access_token}
        try:
            response = self.session.request(
                "GET",
                FACEBOOK_GRAPH_URL + self.version + "/me",
                params=args,
                timeout=self.timeout,
                proxies=self.proxies,
            )
        except requests.HTTPError as e:
            response = json.loads(e.read())
            raise GraphAPIError(response)

        try:
            headers = response.headers
            version = headers["facebook-api-version"].replace("v", "")
            return str(version)
        except Exception:
            raise GraphAPIError("API version number not available")