Add package install test #324
Description
Currently our package installation test on CI checks that we can install the package, but in an environment where we've already installed twine and a bunch of other things. This meant that we missed the dependency on the packaging package in #322.
We should have a separate test in CI that tests that we can install the package in a fresh venv. I'm not sure how to encode the dependency on having already built the wheel. Ideally we'd also test installation with pipx, as this is a good application for it.