Include tests in PyPI tarball

[dotlambda]

This is useful for downstream distributions to test if the package works correctly with their build recipe. Also, it provides a way to test if the dependencies as they are packaged by the distribution are compatible.

[patrys]

I think it would be better to make httmock a module and put tests there to avoid polluting the python namespace.

[koobs]

@patrys Alternatively, one can exclude tests.by with find_packages [1] which many projects use, but making httpmock a module and putting tests inside that is the more robust option.

[1] https://setuptools.readthedocs.io/en/latest/setuptools.html#using-find-packages