* Massively simplify the internal _threading.py by removing everything we don't use.

Enable coverage testing for Python 3 and some for PyPy.

* PyPy can't handle coverage :(

* Account for a racy test and tweak the makefile to ignore errors uploading to coveralls. Python 3.7 was seen to generate that: https://travis-ci.org/gevent/gevent/jobs/334831358

* Manually exclude .so files. They caused coveralls to fail: https://travis-ci.org/gevent/gevent/jobs/334839765#L1755

* Enable coverage on pypy too.

* coverage pragmas

* Skip a test that fails under coverage sometimes