spurious build failures caused by ipykernel 6
ipykernel 6 now captures stdout/stderr from C/C++ code as well as child processes. This is nice, because when we calls something like `spot.automaton("bogus-command|") we should now see the errors, and that will probably also help us debugging C++ code.
However this difference with older kernel is causing python/_autparserr.ipynb
to fail (currently on Arch Linux only, since Debian is still using ipykernel 5). Probably we should redirect stderr to /dev/null explicitly.