When private key is in pkcs#1 format, rcgen::KeyPair::from_der() will return RcgenError::CouldNotParseKeyPair.
Could we add :

        } else if let Ok(rsakp) = RsaKeyPair::from_der(pkcs8) {
            (KeyPairKind::Rsa(rsakp, &signature::RSA_PKCS1_SHA256), &PKCS_RSA_SHA256)

in KeyPair's from_raw in order to support pkcs#1 ?