Use PyString_CheckExact() in crypto_PKCS12_set_friendlyname().  Jean-Paul's find.
diff --git a/src/crypto/pkcs12.c b/src/crypto/pkcs12.c
index cbc874a..26d710f 100644
--- a/src/crypto/pkcs12.c
+++ b/src/crypto/pkcs12.c
@@ -211,8 +211,8 @@
         kwlist, &name))
         return NULL;
 
-    if (name != Py_None && ! PyString_Check(name)) { 
-        PyErr_SetString(PyExc_TypeError, "name must be a string or None");
+    if (name != Py_None && ! PyString_CheckExact(name)) { 
+        PyErr_SetString(PyExc_TypeError, "name must be a str or None");
         return NULL;
     }