Support colocated unit tests

[jayvdb]

See ryanhiebert/backports.csv#33 (comment)

As I understand it, @ryanhiebert wants foo/config.py to be tested by foo/test_config.py , and the tests to be included in the sdist, but not be included in the wheel.

I suspect this is probably possible to be done using stock setuptools with a custom py_modules that includes every module, and a MANIFEST.in which finds all of the tests.

[ryanhiebert]

That's the basic idea, although I name the test file more like foo/config_test.py to allow the two files to be next to each other in my file browser. If there were a good way to have the unittests in the same file, the way rust does, I'd also be interested in that. The closer to the code it's testing, the better, basically.

[chfw]

I am on react side too. It does allow me to find test quicker.