CI: add Python 3.8 & 3.9 builds on arm64 macOS

[Xmader]

actions/setup-python now added prebuilt Python 3.8 & 3.9 packages for arm64 macOS.

See https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

{
    "version": "3.8.10",
    "stable": true,
    "release_url": "https://github.com/actions/python-versions/releases/tag/3.8.10-8879978422",
    "files": [
      {
        "filename": "python-3.8.10-darwin-arm64.tar.gz",
        "arch": "arm64",
        "platform": "darwin",
        "download_url": "https://github.com/actions/python-versions/releases/download/3.8.10-8879978422/python-3.8.10-darwin-arm64.tar.gz"
      },
      ...

{
    "version": "3.9.13",
    "stable": true,
    "release_url": "https://github.com/actions/python-versions/releases/tag/3.9.13-8879985561",
    "files": [
      {
        "filename": "python-3.9.13-darwin-arm64.tar.gz",
        "arch": "arm64",
        "platform": "darwin",
        "download_url": "https://github.com/actions/python-versions/releases/download/3.9.13-8879985561/python-3.9.13-darwin-arm64.tar.gz"
      },
      ...

Previously there were unavailable.

The version '3.8' with architecture 'arm64' was not found for macOS.