Restore SIGPIPE handler to DFL on POpen

Python sets the SIGPIPE handler to SIG_IGN. On execv() signal handlers are
reset to their defaults, EXCEPT those that are SIG_IGN which are left ignored.
In Python 3 subprocess.popen explicitly resets the SIGPIPE handler to SIG_DFL,
but unfortunately in Python 2.7 it does not. This leads to subprocesses being
executed with SIGPIPE ignored. This is often a problem with bash scripts which
rely on SIGPIPE to terminate commands in a pipe, but can easily be a problem
with other applications.

This implements the Python 3 behaviour for Python 2.7 by using a preexec_fn.

changelogs/fragments/restore_sigpipe_dfl.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - Restore SIGPIPE to SIG_DFL when creating subprocesses to avoid it being ignored under Python 2.