which('sh') isn't correct here. 'which' searches in all PATH environ parts. However the shell must be looked up in CS_PATH only.
From man sh(1posix):
> Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH , ensuring that the returned pathname is an absolute pathname and not a shell built-in.
'getconf PATH' queries confstr(_CS_PATH).
I suggest that you modify the POSIX part to:
path = os.confstr("CS_PATH")
which('sh', path=path)