Fix showing SystemExit exception raise inside except handler (#14503)

Doing something like this:

```python
try:
    5 / 0
except Exception as e:
    raise SystemExit
```

was hitting an error inside UltraTB, creating a long traceback of its
internals (which, ironically, UltraTB itself then displays correctly
:-).

`ListTB.get_exception_only()` calls the `ListTB.structured_traceback()`
method *specifically* - even if `self` is a subclass, it won't use the
subclass's method. However, the exception chaining in that method uses
recursion by calling `self.structured_traceback()`, which will use a
subclass's method. Tuples were added as an option there to support
exception chaining, but not all of the machinery in connected classes
expects a tuple.

This just skips the exception chaining logic for the `etb=None` case,
when we're showing the exception only. I'm not sure this is necessarily
the best fix, but I didn't want to spend too much time following code
around a module that's old enough to vote.

Closes  #12104

IPython/core/tests/test_ultratb.py

@@ -298,6 +298,13 b' except Exception:'
     raise ValueError("Yikes") from None
     """
 
+    SYS_EXIT_WITH_CONTEXT_CODE = """
+try:
+    1/0
+except Exception as e:
+    raise SystemExit(1)
+    """
+
     def test_direct_cause_error(self):
         with tt.AssertPrints(["KeyError", "NameError", "direct cause"]):
             ip.run_cell(self.DIRECT_CAUSE_ERROR_CODE)
@@ -306,6 +313,11 b' except Exception:'
         with tt.AssertPrints(["KeyError", "NameError", "During handling"]):
             ip.run_cell(self.EXCEPTION_DURING_HANDLING_CODE)
 
+    def test_sysexit_while_handling_error(self):
+        with tt.AssertPrints(["SystemExit", "to see the full traceback"]):
+            with tt.AssertNotPrints(["another exception"], suppress=False):
+                ip.run_cell(self.SYS_EXIT_WITH_CONTEXT_CODE)
+
     def test_suppress_exception_chaining(self):
         with tt.AssertNotPrints("ZeroDivisionError"), \
              tt.AssertPrints("ValueError", suppress=False):

IPython/core/ultratb.py

@@ -552,28 +552,31 b' class ListTB(TBTools):'
         lines = ''.join(self._format_exception_only(etype, evalue))
         out_list.append(lines)
 
-        exception = self.get_parts_of_chained_exception(evalue)
+        # Find chained exceptions if we have a traceback (not for exception-only mode)
+        if etb is not None:
+            exception = self.get_parts_of_chained_exception(evalue)
 
-        if exception and (id(exception[1]) not in chained_exc_ids):
-            chained_exception_message = (
-                self.prepare_chained_exception_message(evalue.__cause__)[0]
-                if evalue is not None
-                else ""
-            )
-            etype, evalue, etb = exception
-            # Trace exception to avoid infinite 'cause' loop
-            chained_exc_ids.add(id(exception[1]))
-            chained_exceptions_tb_offset = 0
-            out_list = (
-                self.structured_traceback(
-                    etype,
-                    evalue,
-                    (etb, chained_exc_ids),  # type: ignore
-                    chained_exceptions_tb_offset,
-                    context,
+            if exception and (id(exception[1]) not in chained_exc_ids):
+                chained_exception_message = (
+                    self.prepare_chained_exception_message(evalue.__cause__)[0]
+                    if evalue is not None
+                    else ""
+                )
+                etype, evalue, etb = exception
+                # Trace exception to avoid infinite 'cause' loop
+                chained_exc_ids.add(id(exception[1]))
+                chained_exceptions_tb_offset = 0
+                out_list = (
+                    self.structured_traceback(
+                        etype,
+                        evalue,
+                        (etb, chained_exc_ids),  # type: ignore
+                        chained_exceptions_tb_offset,
+                        context,
+                    )
+                    + chained_exception_message
+                    + out_list
                 )
-                + chained_exception_message
-                + out_list)
 
         return out_list