pyflyby-dbg: Support chained exceptions

This commit adds support of stepping through chained exceptions with
pyflyby debugger.

```
$ cat chained_exc.py
-------------------------------------------------------------
def inner():
    raise ValueError("inner")

def outer():
    try:
        inner()
    except Exception as e:
        raise ValueError("outer") from e

if __name__ == "__main__":
    outer()

--------------------------------------------------------------------------
$ py /u/anis/scratch/chained_exc.py
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
File ~/scratch/chained_exc.py:6, in outer()
      5 try:
----> 6     inner()
      7 except Exception as e:

File ~/scratch/chained_exc.py:2, in inner()
      1 def inner():
----> 2     raise ValueError("inner")

ValueError: inner

The above exception was the direct cause of the following exception:

ValueError                                Traceback (most recent call last)
File ~/scratch/chained_exc.py:11
      8         raise ValueError("outer") from e
     10 if __name__ == "__main__":
---> 11     outer()

File ~/scratch/chained_exc.py:8, in outer()
      6     inner()
      7 except Exception as e:
----> 8     raise ValueError("outer") from e

ValueError: outer
> /u/anis/scratch/chained_exc.py(8)outer()
      6         inner()
      7     except Exception as e:
----> 8         raise ValueError("outer") from e
      9
     10 if __name__ == "__main__":

ipdb> exceptions
    0 ValueError('inner')
>   1 ValueError('outer')
```

The support uses native Ipython's debugger or python's pdb support for
chained exceptions and is available with `exceptions` command under
ipdb/pdb command prompt.

Implementation notes
=============================================================

Starting Py3.13, pdb.interaction() supports chained exceptions in case
exception (and not traceback) is specified. This support is backported
to IPython8.16 for earlier Python versions. So the conditions where
chained exceptions won't be supported from pyflyby's debugger would be with Python
version < 3.13 and ipython is not being installed, or IPython's version is
lesser than 8.16.

Ref: PyInf#14001
Reviewer: dhamodha

lib/python/pyflyby/_dbg.py

@@ -324,7 +324,27 @@ def _debug_exception(*exc_info, **kwargs):
             # keep the process waiting for debugger to attach.
             pdb.postloop = _prompt_continue_waiting_for_debugger
         print_verbose_tb(*exc_info)
-        pdb.interaction(None, exc_info[2])
+        # Starting Py3.13, pdb.interaction() supports chained exceptions in case
+        # exception (and not traceback) is specified. This support is backported
+        # to IPython8.16 for earlier Python versions. So the conditions where
+        # chained exceptions won't be supported from here would be with the
+        # Python version < 3.13 and ipython not installed, or IPython's version
+        # is lesser than 8.16.
+        tb_or_exc = exc_info[2]
+        if sys.version_info < (3, 13):
+            # Check if the instance is of IPython's Pdb and its version.
+            try:
+                import IPython
+                if IPython.__version__ >= '8.16':
+                    from IPython.core.debugger import Pdb as IPdb
+                    # This is expected to be True, hence just a safe check.
+                    if isinstance(pdb, IPdb):
+                        tb_or_exc = exc_info[1]
+            except ImportError:
+                pass
+        else:
+            tb_or_exc = exc_info[1]
+        pdb.interaction(None, tb_or_exc)
 
 
 def _debug_code(arg, globals=None, locals=None, auto_import=True, tty="/dev/tty"):