Use PyBytes_ for 3.x compatibility
diff --git a/OpenSSL/crypto/x509.c b/OpenSSL/crypto/x509.c
index 4f29b97..9ee28e8 100644
--- a/OpenSSL/crypto/x509.c
+++ b/OpenSSL/crypto/x509.c
@@ -543,7 +543,7 @@
         PyErr_SetString(PyExc_ValueError, "Undefined signature algorithm");
         return NULL;
     }
-    return PyString_FromString(OBJ_nid2ln(nid));
+    return PyBytes_FromString(OBJ_nid2ln(nid));
 }