> I have to replace _PyOS_URandom with a variant that doesn't need the GIL

Please don't replace it, but add a new function which report errors differently. Which kind of granularity do you expect for the error reporting? Just a boolean (success or failure)?

Most implementations of _PyOS_URandom() already has a private "int raise" parameter to specify how errors are reported: raise an exception or call Py_FatalError().