Add automated checks for supported language versions/toolchains

[jasongraffius]

Actions and checks on pull requests should at least check the earliest supported Python version, so that new pull requests don't break our version compatibility goals. Additionally, testing with the latest stable Python release and various toolchains for the backends (clang, gcc, etc.) would be useful as well to ensure proper support is maintained.

[jasongraffius]

Note, #165 updates the workflow to build the C++ backend in both GCC and clang toolchains, rather than whatever default is configured in the runner base (gcc).