All touched Python scripts already assume and require UTF8, so manually
specifying encoding or decoding for functions in the subprocess module
is redundant to just using text=True, which exists since Python 3.7
We would only modify the parent process' first --tmpdir arg.
Now we tack on an additional --tmpdir after the parent's arguments. Also simplifies the code.
Co-authored-by: Ryan Ofsky <ryan@ofsky.org>
