Backport of PEP 654 (exception groups)

This is a backport of the BaseExceptionGroup and ExceptionGroup classes from Python 3.11. It contains the following: The exceptiongroup.BaseExceptionGroup and exceptiongroup.ExceptionGroup classes A utility function (exceptiongroup.catch()) for catching exceptions possibly nested in an exception group Patches to the TracebackException class that properly formats exception groups (installed on import) An exception hook that handles formatting of exception groups through TracebackException (installed on import) Special versions of some of the functions from the traceback module, modified to correctly handle exception groups even when monkey patching is disabled, or blocked by another custom exception hook: traceback.format_exception() traceback.format_exception_only() traceback.print_exception() traceback.print_exc() If this package is imported on Python 3.11 or later, the built-in implementations of the exception group classes are used instead, TracebackException is not monkey pat