Exceptiongroup

- Added custom versions of several ``traceback`` functions that work with exception
groups even if monkey patching was disabled or blocked

- Don't monkey patch anything if ``sys.excepthook`` has been altered
- Fixed formatting of ``SyntaxError`` in the monkey patched
``TracebackException.format_exception_only()`` method

- **BACKWARDS INCOMPATIBLE** Changed ``catch()`` to not wrap an exception in an
exception group if only one exception arrived at ``catch()`` and it was not matched
with any handlers. This was to match the behavior of ``except*``.

- **BACKWARDS INCOMPATIBLE** Changed ``catch()`` to match the behavior of ``except*``:
each handler will be called only once per key in the ``handlers`` dictionary, and with
an exception group as the argument. Handlers now also catch subclasses of the given
exception types, just like ``except*``.

- Patch for ``traceback.TracebackException.format_exception_only()`` (PR by Zac Hatfield-Dodds)

- Update `PEP 678`_ support to use ``.add_note()`` and ``__notes__`` (PR by Zac Hatfield-Dodds)