Test the dists

[atugushev]

When you run tox, it works with the source in the project dir and even creates some temporary dists. But they are not what you're going to publish to PyPI hence you test not exactly the same as users will get. Also, you may be testing not what's installed for various reasons.
Better way would be to divide this process into stages:

  (1) build dists -> (2) test dists from [1] -> (3) upload *exactly* those dists from [1] that have been tested

Originally posted by @webknjaz in #1012 (comment)