Unclosed resource warning for /dev/null in PEX teardown.

[jsirois]

This presents as:

02:44:24.35 [WARN] sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='w' encoding='UTF-8'>

This is unfortunate since we really do intend to leave that filehandle open until Python tears it down :/:
https://github.com/pantsbuild/pex/blob/47047749f52d80f16dd8b4fd940e76c0c110fdd0/pex/pex.py#L458-L464

Fixing the warning will require warnings management or giving up on this robust way to /dev/null teardown and returning to the hackier construction of a DevNull file-like object as was done previously.