Set sys.last_value / sys.last_exc before entering the debugger (#380)

**Background:**
In commit 6eae532, we introduced a new utility in pyflyby named `saveframe`. This utility is designed to save error stack frames upon an exception. Users can also enter a debugger after an exception (using ipdb.pm()) and call `saveframe` to dump a specific frame.

`saveframe` relies on `sys.last_value` (or `sys.last_exc` in Python 3.12) to retrieve the last exception object raised. These attributes are automatically set by Python after an uncaught exception occurs.

**Issue:**
When a user executes a script or command using `py <some_script_or_command>` or employs the `@debug_on_exception` decorator, the exception is caught, and the debugger is invoked. Consequently, `sys.last_value` (or `sys.last_exc`) is not set. If the user attempts to call `saveframe` within the debugger, it fails because the necessary exception information is unavailable.

**Solution:**
In this commit, we modified the code to explicitly set `sys.last_value` (or `sys.last_exc`) after an exception is caught and before entering the debugger. This ensures that users can directly enter the debugger using `py`, and successfully call the `pyflyby.saveframe` function.

Reviewer: rakh-deshaw
Request: PyInf#12047

lib/python/pyflyby/_dbg.py

@@ -316,6 +316,14 @@ def _debug_exception(*exc_info, **kwargs):
         # will cause print_verbose_tb to include a line with just a colon.
         # TODO: avoid that line.
         exc_info = ("", "", exc_info)
+    if exc_info[1]:
+        # Explicitly set sys.last_value / sys.last_exc to ensure they are available
+        # in the debugger. One use case is that this allows users to call 
+        # pyflyby.saveframe() within the debugger.
+        if sys.version_info < (3, 12):
+            sys.last_value = exc_info[1]
+        else:
+            sys.last_exc = exc_info[1]
 
     with _DebuggerCtx(tty=tty) as pdb:
         if debugger_attached: