Fix critical bug when re-entering IPython 8.0.0 (and later) external shell (#533)

* Fix critical bug when re-entering IPython 8.4.0+ external shell

- IPython 8.4.0+ performs at-exit operations after calling mainloop
  (introduced by 08d54c0e367b535fd88aca5273fd09e5e70d08f8 commmit),
  breaking necessary PuDB functionality. This prevents re-entering
  IPython shell
- Quick fix sets/unsets internal shell._atexit_once_called property
  during run_ipython_shell_v11, preventing those operations from being
  executed

* Fix "E501 line too long" error from flake8

- Bug start in IPython 8.0.0. Update comment accordingly.

Co-authored-by: Vladyslav Sydorov <vlad@vsydorov.com>