I have doubts about combining setschedpolicy and setschedparam.

On one hand, they are set by independent functions in C and have separate flags. When pass arguments separately we don't need to invent a parameter name: just remove the common "posix_spawnattr_" prefix from corresponding C function names. Passing scheduler=(None, param) looks clumsy. And from implementation side, parsing separate arguments is easier that parsing a single argument as a 2-tuple.

On other hand, these arguments are not completely independent: the setschedparam argument is required if setschedpolicy is specified.

Passing as separate arguments still looks more preferable to me.