After looking into this:

https://bugzilla.suse.com/show_bug.cgi?id=917759

DimStar detected a difference in the way the constants are exported in the new
version of SWIG (3.0.5):

The relevant portion is here:

+__m2crypto.PKCS5_SALT_LEN_swigconstant(__m2crypto)
 PKCS5_SALT_LEN = __m2crypto.PKCS5_SALT_LEN
+SWIGINTERN PyObject *PKCS5_SALT_LEN_swigconstant(PyObject
*SWIGUNUSEDPARM(self), PyObject *args) {
+  SWIG_Python_SetConstant(d, "PKCS5_SALT_LEN",SWIG_From_int((int)(8)));
+     { (char *)"PKCS5_SALT_LEN_swigconstant", PKCS5_SALT_LEN_swigconstant,
METH_VARARGS, NULL},
-  SWIG_Python_SetConstant(d, "PKCS5_SALT_LEN",SWIG_From_int((int)(8)));

The symbol exported is PKCS5_SALT_LEN_swigconstant now in the m2 module.