[CT-653] [Feature] Allow running unit/integration tests locally with any supported Python version

[jwills]

Is this your first time opening an issue?

• I have read the expectations for open source contributors

Describe the Feature

This is more of a rough edge for a non-professional dbt-core developer: the Makefile hard-codes py38 as the preferred python version in tox for running unit tests and integration tests; we use Python 3.9 at my day job, so when I need to do work on dbt-core I need to remember to swap out the version in the Makefile to py39 before I run dbt tests and then switch it back before I send a PR upstream.

Describe alternatives you've considered

I would love it if the Makefile was either a) configured to work with the Python version that I had installed locally as long as it was supported (i.e., if the default in the Makefile was py and py-integration instead of py38 and py38-integration), or, if that presents a problem for e.g. CI or some such thing, if it was easy (like via an environment variable that was documented in the CONTRIBUTING.md guide) for me to override the setting in the Makefile on my local machine.

Who will this benefit?

Developers who want to create PRs for dbt-core who do not work at dbt Labs.

Are you interested in contributing this feature?

Happy to!

Anything else?

Issue originally discussed in a Slack thread in dbt-core-development: https://getdbt.slack.com/archives/C50NEBJGG/p1652466095907019

[jtcohen6]

Using py and py-integration makes tons of sense—it will just uses your own local PATH version of Python, right?

We don't use the Makefile in CI (we hit tox directly), so I don't think there should be any implications there. Let's do a nice thing for non-professional dbt-core developers :)

The only edge I could see is someone very eagerly upgrading to a new minor version of Python, before dbt-core officially supports it, e.g. Python 3.10 prior to the v1.1 release. But I'd like to see us supporting new Python minor versions more proactively going forward, especially when they promise to have some good stuff inside...

@jwills proceed at your leisure!