Avoid unwanted behavior change in shlex.quote (see #9723).

Avoid unwanted behavior change in shlex.quote (see #9723).

I simplified the quote code to use a regex instead of a loop+test when I
moved pipes.quote to shlex in 5966eeb0457d; Ezio Melotti pointed out
that my regex contained redundant parts (now removed) and allowed
non-ASCII characters (now disallowed).

I think common UNIX shells don’t quote non-ASCII characters, but there’s
no harm in doing so.  We’ll see if users request a change.