Include all test files in sdist

[booxter]

This is useful to run unit tests during distro builds.

[CLAassistant]

All committers have signed the CLA.

[baberabb]

Hi! can you explain a bit more of how this will be useful. Also hoping to push a new release next week!

[booxter]

This is useful to run unit tests during distro builds. Then distros can pull your package from PyPI instead of GitHub. During build, they may want to run your test suite to make sure nothing is broken in terms of dependencies etc. Right now, distributions have to deal with Git to make sure tests are executed.