Add support for running tests via pytest-runner.

--HG--
extra : amend_source : d60530fbb3e3cc3455abae8e5100f85f697f3dac
Use pytest for running tests in Travis
Bypass the 'manual test' when running with pytest
Bypass shlib_test when running tests under pytest
Fix build of setuptools on Windows (do not create empty tag)
Force sdist zip format (preferred for _build_egg)

--HG--
extra : amend_source : eb261716fc15148a101c059fb03c0f51b683c673
Remove test_build_egg. The test doesn't work and it's difficult to test _build_egg given the current interface.
Remove test_do_download, which also no longer works due to complexities around secure downloaders.