Test pickling a simple callable (does not work). (#3906)

* Test pickling a simple callable (does not work).

Currently only documents that it does not work. Starting point for future fix.

* Use re.search to accommodate variations of the TypeError message.

* PyPy: exercise full dumps/loads cycle.

* Adding explicit "broken" comment.

tests/test_pickling.cpp

@@ -61,6 +61,8 @@ void wrap(py::module m) {
 } // namespace exercise_trampoline
 
 TEST_SUBMODULE(pickling, m) {
+    m.def("simple_callable", []() { return 20220426; });
+
     // test_roundtrip
     class Pickleable {
     public: