[3.6] bpo-30703: Improve signal delivery (GH-2415) (#2527)

[3.6] bpo-30703: Improve signal delivery (GH-2415) (#2527)

* [3.6] bpo-30703: Improve signal delivery (GH-2415)

* Improve signal delivery

Avoid using Py_AddPendingCall from signal handler, to avoid calling signal-unsafe functions.

* Remove unused function

* Improve comments

* Add stress test

* Adapt for --without-threads

* Add second stress test

* Add NEWS blurb

* Address comments @haypo.
(cherry picked from commit c08177a1ccad2ed0d50898c2731b518c631aed14)

* bpo-30796: Fix failures in signal delivery stress test (#2488)

* bpo-30796: Fix failures in signal delivery stress test

setitimer() can have a poor minimum resolution on some machines,
this would make the test reach its deadline (and a stray signal
could then kill a subsequent test).

* Make sure to clear the itimer after the test