Mac builds with Python 3.6 are failing in Github Actions

[league]

The transcript says:

  Version 3.6 was not found in the local cache
  Error: The version '3.6' with architecture 'arm64' was not found for macOS 14.4.1.
  The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

That list does contain some 3.6.x, but then I noticed the part about "architecture arm64"… so this comment seems to be relevant:

For me this looks like it's probably that the macos-latest Github action runner appears to have switched to an M1 ARM mac, where at least last week macos-latest was an x86 mac.

So I see a couple of options:

• Pin macos-latest to an earlier revision so we're still ensuring x86, or
• Drop 3.6 from CI for mac. It's already excluded for self-hosted, but we're still using it on Ubuntu.

[league]

This should be ready, and can prevent other branches from failing on macos/py36 if merged or cherry-picked there.