Upgrade GHA workflow to use v2 of the official actions

[webknjaz]

(https://twitter.com/webKnjaZ/status/1261678107439792129 follow-up)

So from what I saw, current workflows use actions/setup-python and actions/checkout that are v1 but both have v2 available.

It should be a low hanging fruit for somebody who wants to send an easy beginner-level PR.

[henryiii]

v2 of setup python should be trivial (it basically just allows you to pin to any Python version), but v2 of checkout is probably a bit of a challenge to do nicely - it does not include the information needed for setuptools_scm, even with depth=0, so you have to unshallow, which is (IMO) uglier than using "v1" for now. Here's the issue: actions/checkout#249 .