gh-106670: Allow Pdb to move between chained exceptions#106676

This lets Pdb receive an exception, instead of a traceback, and whenthis is the case and the exception are chained, the new `exceptions` commandallows to both list (no arguments) and move between the chained exceptions.That is to say if you have something like    def out():        try:            middle()                                # B        except Exception as e:            raise ValueError("foo(): bar failed")   # A    def middle():        try:            return inner(0)                         # D        except Exception as e:            raise ValueError("Middle fail") from e  # C    def inner(x):        1 / x                                       # EOnly A was reachable after calling `out()` and doing post mortem debug.With this all A-E points are reachable with a combination of up/down,and ``exception <number>``.This also change the default behavior of ``pdb.pm()``, as well as`python -m pdb <script.py>` to receive `sys.last_exc` so that chainedexception navigation is enabled.We do follow the logic of the ``traceback`` module and handle the``_context__`` and ``__cause__`` in the same way. That is to say, we try``__cause__`` first, and if not present chain with ``__context__``. Inthe same vein, if we encounter an exception that has``__suppress_context__`` (like when ``raise ... from None``), we do stopwalking the chain.Some implementation notes: - We do handle cycle in exceptions - cleanup of references to tracebacks are not cleared in ``forget()``, as   ``setup()`` and ``forget()`` are both for setting a single   exception. - We do not handle sub-exceptions of exception groups.Closesgh-106670