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)

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 patched and the exception hook won't be
installed.