Fix a compilation warning with python2.4
Fix a compilation warning with python2.4

It seems python2.4 expects the format argument to
PyArg_ParseTupleAndKeywords to be "char*" and thus the compiler emits a
warning when passing "const char*" (python2.5 is betteri here). Remove
the const from format in ACL_init to fix this.

iustin/pylibacl,iustin/pylibacl