<https://earthly.dev> is an open source (MPL-2.0) project for defining
and executing builds using containers.

It meets a lot of the same needs as the Vagrant boxes that are in this
project using containers rather than VMs.

The advantage of pairing this with GitHub Actions is that it allows
maintainers and contributors to reproduce test results locally which is
very difficult when using GitHub Actions natively.

stdeb interacts with a lot of system-level utilities so having
convenient testing across multiple supported distributions is highly
valuable.

Since we have the earthly configuration for local testing, we may as
well use it for GitHub Actions as well.

Use the matrix support to pass the OS build argument to Earthly.

Keep platforms that aren't working yet disabled.

* Use python3 for running packaging tests.

* Update version of psycopg2 used for tests.
  I got build failures on bullseye with the earlier version but success
  with this last one (the psycopg project apparently returned to the
  psycopg module for v3).

Right now this only sets a preferred line-length limit.
It's possible this will be folded into a pyproject.toml as part of other
updates to the project.

Travis CI has long since stopped providing value to open source projects
and I believe that the Earthly builds cover the same local testing niche
as the Vagrant files, which I was not able to successful update.

If another maintainer or contributor wants to restore the Vagrant
support I will help as I can, but I don't have a working vagrant
provider locally.

Avoid copying in .git and other unecessary files by specifying source
files explicitly.

To make this slightly less unfun factor it into a function.

With the addition of #203, a missing Python 2 binary will raise an error
instead of assuming that the binary is called `python`. As a result,
this test script now fails on platforms where no Python 2 installation
is available.