stash nanobind data structures in interpreter state dictionary

Previously, nanobind stored its internal data structures in the
``builtins`` dictionary, which was rightfully criticised in issue #96.
This tentative commit instead uses the interpreter state dictionary
exposed via ``PyInterpreterState_GetDict()``.

src/nb_internals.cpp

@@ -374,9 +374,13 @@ static void internals_make() {
 
  internals_p = new nb_internals();
 
+ PyObject *dict = PyInterpreterState_GetDict(PyInterpreterState_Get());
+ if (!dict)
+ fail("nanobind::detail::internals_make(): PyInterpreterState_GetDict() failed!");
+
  PyObject *capsule = PyCapsule_New(internals_p, nullptr, nullptr);
  PyObject *nb_module = PyModule_NewObject(nb_name.ptr());
- int rv = PyDict_SetItemString(PyEval_GetBuiltins(), NB_INTERNALS_ID, capsule);
+ int rv = PyDict_SetItemString(dict, NB_INTERNALS_ID, capsule);
  if (rv || !capsule || !nb_module)
  fail("nanobind::detail::internals_make(): allocation failed!");
  Py_DECREF(capsule);
@@ -480,13 +484,16 @@ static void internals_make() {
 }
 
 static void internals_fetch() {
- PyObject *capsule =
- PyDict_GetItemString(PyEval_GetBuiltins(), NB_INTERNALS_ID);
+ PyObject *dict = PyInterpreterState_GetDict(PyInterpreterState_Get());
+ if (!dict)
+ fail("nanobind::detail::internals_fetch(): PyInterpreterState_GetDict() failed!");
+
+ PyObject *capsule = PyDict_GetItemString(dict, NB_INTERNALS_ID);
 
  if (capsule) {
  internals_p = (nb_internals *) PyCapsule_GetPointer(capsule, nullptr);
  if (!internals_p)
- fail("nanobind::detail::internals_fetch(): internal error!");
+ fail("nanobind::detail::internals_fetch(): capsule pointer is NULL!");
  return;
  }