Get rid of PYTHONPATH mangling

[Tristan Seligmann]

A few pieces of code that launch a child process try to build a PYTHONPATH environment variable by stitching sys.path together; this produces incorrect results, depending on what happens to be on sys.path. It is also unnecessary: if the current value of PYTHONPATH resulted in the current value of sys.path, then it should have exactly the same effect in the child process without any modification.