def _validate_proxy_scheme_url_selection(self, url_scheme):
        """
        Validates that were not attempting to do TLS in TLS connections on
        Python2 or with unsupported SSL implementations.
        """
        if self.proxy is None or url_scheme != "https":
            return

        if self.proxy.scheme != "https":
            return

        if six.PY2 and not self.proxy_config.use_forwarding_for_https:
            raise ProxySchemeUnsupported(
                "Contacting HTTPS destinations through HTTPS proxies "
                "'via CONNECT tunnels' is not supported in Python 2"
            )

        Validates that were not attempting to do TLS in TLS connections on
        Python2 or with unsupported SSL implementations.
        
lib/urllib3/poolmanager.py