Will the following syntax not work for your use case?

with:
    python-version: '3.9.0-alpha - 3.9.0' # SemVer's version range syntax

Currently there is Python 3.9.0b5 but lets say version Python 3.9.0b6 comes out and then finally Python 3.9.0 officially comes out.

By specifying the range, the latest pre-release version will always be pulled. Once a stable version of Python 3.9 comes out (no new pre-release versions should come out for that specific version), the stable version of Python would be used.

Thank you for the quick response and for pointing out the solution. I now see it was right there in the docs. My apologies for not finding it there myself.

I ended up with 3.9.0-alpha - 3.9.X to get 'all prerelease or release' and implemented that as follows.

https://github.com/altendky/qtrio/blob/2f041af7a346b91d5e8f0765ee33c86cdc6f7e50/.github/workflows/ci.yml#L95

        python: ['3.6', '3.7', '3.8', '3.9']

https://github.com/altendky/qtrio/blob/2f041af7a346b91d5e8f0765ee33c86cdc6f7e50/.github/workflows/ci.yml#L104

          python-version: '${{ matrix.python }}.0-alpha - ${{ matrix.python }}.X'

Now just waiting on PySide2 and PyQt5 to release 3.9-supporting wheels...

Actually, I suppose this points out an error (I think) in the readme. The range with the 0 micro/patch version did not (would not sensibly) allow stable releases beyond .0 to be used. As I recall just major.minor didn't work either and I had to use the major.minor.X noted above.

See also #213.