def urlopen(  # type: ignore[override]
        self, method: str, url: str, redirect: bool = True, **kw: typing.Any
    ) -> BaseHTTPResponse:
        "Same as HTTP(S)ConnectionPool.urlopen, ``url`` must be absolute."
        u = parse_url(url)
        if not connection_requires_http_tunnel(self.proxy, self.proxy_config, u.scheme):
            # For connections using HTTP CONNECT, httplib sets the necessary
            # headers on the CONNECT to the proxy. If we're not using CONNECT,
            # we'll definitely need to set 'Host' at the very least.
            headers = kw.get("headers", self.headers)
            kw["headers"] = self._set_proxy_headers(url, headers)

        return super().urlopen(method, url, redirect=redirect, **kw)
Same as HTTP(S)ConnectionPool.urlopen, ``url`` must be absolute.
src/urllib3/poolmanager.py