$ python3.6
Python 3.6.3 (default, Oct 27 2017, 17:16:29) 
[GCC 5.4.0] on netbsd8
Type "help", "copyright", "credits" or "license" for more information.
>>> import libxml2
Traceback (most recent call last):

+ Trace 238145

>>> 

Not surprising that PyCObject_Check isn't found. It is introduced in libxml2/python/libxml_wrap.h:

#ifndef PyCapsule_New
#define PyCapsule_New PyCObject_FromVoidPtrAndDesc
#define PyCapsule_CheckExact PyCObject_Check
#define PyCapsule_GetPointer(o, n) PyCObject_GetDesc((o))
#endif

In Python-3.6.3/Include/pycapsule.h:

PyAPI_FUNC(PyObject *) PyCapsule_New(
    void *pointer,
    const char *name,
    PyCapsule_Destructor destructor);

so as far as I can tell, PyCapsule_New is a real function, and not a macro, so #ifndef PyCapsule_New will fire.

Also:

#define PyCapsule_CheckExact(op) (Py_TYPE(op) == &PyCapsule_Type)

so PyCapsule_CheckExact, which exists, then gets redefined by libxml_wrap.h to PyCapsule_Check which doesn't, which results in that symbol not being find when trying to load libxml2.