CI: test (3.8) can no longer install Python 3.8 on ubuntu-latest (Ubuntu 26.04), so every Build run fails #26

The test (3.8) job of the Build workflow can no longer install Python 3.8, so every new run of Build fails, on pull requests and on main alike.

.github/workflows/build.yml runs the matrix on runs-on: ubuntu-latest. As of the run on PR #25 (https://github.com/Stephenson-Software/trace-client-python/actions/runs/40644211569), ubuntu-latest resolves to Ubuntu 26.04, and actions/setup-python@v5 fails at the install step:

Version 3.8 was not found in the local cache
##[error]The version '3.8' with architecture 'x64' was not found for Ubuntu 26.04.

The test (3.10) and test (3.12) jobs are then cancelled by the matrix's default fail-fast, so the run carries no signal at all. The last green run on main (2026-10-24, run 40076218721) predates the runner image change, and the code itself is not at fault: the same 67 tests pass on a local Python 3.8.10 at main (a69dc64).

The README promises Python 3.8+, and the 3.8 job is the only thing that holds that floor, so it should not be dropped and requires-python should not be raised to work around this.

Possible fixes, for the owner to choose between (the workflow is a do-not-auto-merge path for the dev loop):

• Pin the 3.8 matrix entry to a runner image that setup-python still publishes 3.8 builds for (e.g. ubuntu-22.04 or ubuntu-24.04, after confirming in actions/python-versions' versions-manifest.json that the image is listed for 3.8), keeping ubuntu-latest for 3.10 and 3.12.
• Run the 3.8 job in a python:3.8 container instead of relying on setup-python.
• Optionally set fail-fast: false so one unavailable interpreter does not hide the result of the other two.

Until this is fixed, no PR here can satisfy the Build gate, including the 0.4.2 release PR (#25).

This issue was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

drafted by Claude on behalf of Daniel Stephenson