* Trivial cleanups following bpo-31370

* Also cleanup the "importlib._bootstrap_external" module

Use _thread.count() to wait until threads exit. The new context
manager prevents the "dangling thread" warning.

* test_thread.test_forkinthread() now waits until the thread completes.
* Check the status in the test method, not in the thread function
* Don't ignore RuntimeError anymore: since the commit
  346cbd351ee0dd3ab9cb9f0e4cb625556707877e (bpo-16500,
  os.register_at_fork(), os.fork() cannot fail anymore with
  RuntimeError.
* Replace 0.01 literal with a new POLL_SLEEP constant
* test_forkinthread(): test if os.fork() exists rather than testing
  the platform.

test_thread: setUp() now uses support.threading_setup() and
support.threading_cleanup() to wait until threads complete to avoid
random side effects on following tests.

Co-Authored-By:  Grzegorz Grzywacz <grzegorz.grzywacz@nazwa.pl>

I have compared output between pre- and post-patch runs of these tests
to make sure there's nothing missing and nothing broken, on both
Windows and Linux.  The only differences I found were actually tests
that were previously *not* run.