Testing Systems? (this really should be a Discussions thread)

(This really should be a Discussions thread, especially as we don't have a specific mailing list for non-issue discussions)

So, y'all know that I've been testing PRs on the following systems:

- Windows 10: Via PyCharm, via PSCore, in Cygwin
- Ubuntu 18.04: on WSL 1.0, on VirtualBox
- Ubuntu 20.04: on VirtualBox
- FreeBSD 12.2: on VirtualBox

I'm considering dropping the Ubuntu-on-VirtualBox test systems because they practically duplicate the GitHub Actions (GA) test systems anyways. In their place, I'll implement the following:

- OpenSUSE Leap 15.2: on VirtualBox
- AlmaLinux (when it comes out): on VirtualBox

Note: I'll keep the "Ubuntu 18.04 on WSL" test system because tests have conclusively proven that WSL have some "gotchas" that need to be taken into account (e.g., [this test](https://github.com/aio-libs/aiosmtpd/blob/7449778b8994685ac5628d4eb2affea2c70a7541/aiosmtpd/tests/test_server.py#L56-L65) completely fails on WSL).

Do you think I need to implement additional testing systems? Feel free to suggest additional test systems, on one condition: The suggested systems must _NOT_ be a rolling distro, and preferably it has some kind of "LTS" edition.

(OpenSUSE Leap does not seem to have an "LTS" edition, but I'm still including it because [a survey by CloudLinux seem to indicate that quite a sizable number of respondents are thinking of switching to OpenSUSE](https://www.reddit.com/r/ProjectLenix/comments/kd1cqx/more_survey_insights/).)