Include tests in PyPI sdist

[jayvdb]

In order to submit this for inclusion in openSUSE package collection, I need to run the tests offline in the isolated rpmbuild workers. The first step for that is getting the tests into the tarball.

This can be done using a MANIFEST.in, but there are newer solutions depending on how the sdist is built and uploaded to PyPI.

e.g. dephell automatically adds tests to sdist if they are below a certain size.

[jayvdb]

The test suite is a bit limited atm, not surprising as it is still new.

When expanding them, it would be great if a "network" mark was used on all tests which require network, so that I can run pytest -k not network to exclude them.