[BUG] pickleable base C++ classes can incur object slicing if derived typeid is not registered with pybind11

Issue description

Context: https://github.com/pybind/pybind11/pull/2972#discussion_r658848867

If you have C++ base class that is exposed to pybind11, but then you have a derived class that is not exposed to pybind11, then you will encounter object slicing (wikipedia) via a pickle -> unpickle round trip.
Slicing may mean that the unpickled object has the wrong vtable, data, etc.

Reproducible example code

https://github.com/pybind/pybind11/compare/master...EricCousineau-TRI:issue-3062 (derived from fixes in #2972)
commit: https://github.com/pybind/pybind11/commit/7ce1ad059c58554453d42da3e9cd10cad978459a

Upon running the test:

$ make pytest
...
    def test_roundtrip_simple_cpp_derived():
        p = m.make_SimpleCppDerivedAsBase()
        p.num = 404
        assert m.get_value_via_vtable_cpp(p) == 10
...
        data = pickle.dumps(p, pickle.HIGHEST_PROTOCOL)
        p2 = pickle.loads(data)
        assert isinstance(p2, m.SimpleBase)
        assert p2.num == 404
>       assert m.get_value_via_vtable_cpp(p2) == 10
E       assert 1 == 10  # Egad! Error!

\cc @rwgk @YannickJadoul @elkhrt

Start with the linked reproducer and run make pytest, focusing on test_roundtrip_simple_cpp_derived and the make_SimpleCppDerivedAsBase and get_value_via_vtable_cpp entry points. Trace the pickle round trip for an exposed base with an unexposed derived type. Done means the unpickled object preserves the derived vtable behavior and passes the shown assertions.