Add Python 3.8 support about setup-python

[madhurig]

The update is in progress, we expect python 3.8 to be available in the next 2-3 days on the virtual environments for GitHub actions.

[madhurig]

Python 3.8 is now available on windows, macOS and ubuntu virtual environments for GitHub actions. Thank you for your patience.

[damccorm]

@bryanmacfarlane who owns this now

[thoward27]

Is there an ETA for this? Really looking forward to having 3.8 available for my actions.

[hugovk]

@albertus82 Thanks for keeping us updated!

And thanks GitHub for adding it!

(In second place of the CIs I'm checking. 🥇Travis CI: 21h from release to availability; 🥈GHA 21 days; Azure Pipelines and AppVeyor still WIP.)

[albertus82]

At the moment, Python 3.8.0 seems to be available in Windows and Ubuntu, but not yet in macOS: https://github.com/albertus82/jar-classes-version-detector/runs/284581330

[albertus82]

The macOS update apparently took a few more days, but now it seems to be working: https://github.com/albertus82/jar-classes-version-detector/runs/288140878. Thanks.

[webknjaz]

@damccorm ^

[matthewfeickert]

cc @kratsg @lukasheinrich

[loozhengyuan]

FWIW, since it is taking some time for the newer Python versions to be available in the tool-cache, here's a snippet in case anyone requires >=3.8 in their existing workflow. You can use any tags available from Docker Hub:

jobs:
  test:
    name: Test on ${{ matrix.container }}
    runs-on: ubuntu-latest
    container: ${{ matrix.container }}
    strategy:
      matrix:
        container:
          - python:3.8-slim
          - python:3.7-slim
          - python:3.6-slim

    steps:
    - uses: actions/checkout@v1
    - name: Check Python version
      run: |
        python -V