This allows the -j flag to regrtest to propagate warnings settings
properly.

Thanks Matias Bordese. (a few modifications of my own)

  in a `with` statement.  Contributed by Giampaolo Rodola.

in the _posixsubprocess C extension module it would unintentionally close the fds and raise
an error.

in the _posixsubprocess C extension module it would unintentionally close the fds and raise
an error.