The test fails on Python 3 only, and on non-pure builds only.  I added
PYTHONFAULTHANDLER=1 to the environment to make the failure less
mysteriously silent.

I've also updated tox.ini to use extras=foo instead of deps=.[foo] and
to use factors to set PURE_PYTHON=1 instead of defining an entire
environment.  This now lets you run arbitrary pure Python builds like
tox -e py37-pure.