-1 on this one.
It is not a portable decision (only *nix OSes do have /dev/null).
Also, why would we want it as a default constant? The subprocess module
would need to open /dev/null every time. Despite that, I can't see how
would someone use the redirection of errors to /dev/null through a
python script and, at the same time, make it seem not a bad practice at all.