Proxy connection issue?

[mansfieldj3]

When running the below code based on this example, minus the proxy placeholder obvs, I am getting an issue with the network being unreachable, is anyone else encountering this or is this likely to be specific to my environment? Note, when this proxy is set for HTTPS_PROXY and HTTP_PROXY at an environment level I am not encountering this problem

        ctx = ClientContext(url).with_client_certificate(**cert_credentials)
        def set_proxy(request):
            print("Inject proxy settings...")
            proxy = {
                "http": placeholder_proxy,
                "https": placeholder_proxy
            }
            request.proxies = proxy

        ctx.pending_request().beforeExecute += set_proxy

        result = Web.get_context_web_information(ctx).execute_query()
        print(result.value.LibraryVersion)

Error: HTTPSConnectionPool(host='login.microsoftonline.com', port=443): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7f11680899a0>: Failed to establish a new connection: [Errno 101] Network is unreachable')) while establishing connection and retrieving the auth cookie