fix build with -Wcast-function-type -Werror on gcc8

[anarcat]

New versions of gcc8 will fail to build from source on Python
declarations because of the hairy cast we're doing there, example:

pam_python.c:1355:19: error: cast between incompatible function types from 'PyObject * (*)(PyObject *, PyObject *, PyObject *)' {aka 'struct _object * (*)(struct _object *, struct _object *, struct _object *)'} to 'PyObject * (*)(PyObject *, PyObject *)' {aka 'struct _object * (*)(struct _object *, struct _object *)'} [-Werror=cast-function-type]

This fix comes form the upstream cpython implementation:

python/cpython@62be742#diff-c3cf251f16d5a03a9e7d4639f2d6f998