sdist is missing tests

[mtelka]

The sdist package at PyPI is missing tests. Please add tests to sdist to make downstream testing easier. Thank you.

[jaraco]

Note that some tests are included by default, suggesting that tests should be included in sdists. I've come to the opinion that the sdist should closely reflect what's present in the SCM (Git repo), but there are many opinions on the matter.

[webknjaz]

Please add tests to sdist to make downstream testing easier.

Shameless plug: https://github.com/marketplace/actions/checkout-python-sdist — this is an action that helps with making the CI sdist-centric rather than relying on Git.

[bisiabbey24]

#226