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

@@ -552,6 +552,8 b' class ListTB(TBTools):'
         lines = ''.join(self._format_exception_only(etype, evalue))
         out_list.append(lines)
 
+        # 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):
@@ -573,7 +575,8 b' class ListTB(TBTools):'
                     context,
                 )
                 + chained_exception_message
-                + out_list)
+                    + out_list
+                )
 
         return out_list