PyModule_AddObject fix for Python 3.10

[radarhere]

Resolves #5193

The first commit adds python3 -c "from PIL import Image" to .ci/test.sh - for whatever reason, this fails in Python 3.10, while our pytest command does not.

The second commit replaces instances like

#ifdef HAVE_XCB
    PyModule_AddObject(m, "HAVE_XCB", Py_True);
#else
    PyModule_AddObject(m, "HAVE_XCB", Py_False);
#endif

with

    PyObject *have_xcb;
#ifdef HAVE_XCB
    have_xcb = Py_True;
#else
    have_xcb = Py_False;
#endif
    Py_INCREF(have_xcb);
    PyModule_AddObject(m, "HAVE_XCB", have_xcb);

This is per the example in https://docs.python.org/3/c-api/module.html#c.PyModule_AddObject, which calls Py_INCREF before PyModule_AddObject.