feature: fallback to pre-release when no stable version is found (#414)

This allows to specify version like `3.11` or `pypy3.10` in workflows before those versions are released.
This lessen the burden for users of `setup-python` by not having to modify their workflow twice: once when a pre-release is available (e.g. `3.11-dev`) and once when the first stable release is published (e.g. `3.11`)

.github/workflows/test-python.yml

@@ -149,6 +149,35 @@ jobs:
     - name: Run simple code
       run: python -c 'import math; print(math.factorial(5))'
 
+  setup-prerelease-version:
+    name: Setup 3.12 ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [macos-latest, windows-latest, ubuntu-latest]
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+
+    - name: setup-python 3.12
+      id: setup-python
+      uses: ./
+      with:
+        python-version: '3.12'
+        allow-prereleases: true
+
+    - name: Check python-path
+      run: ./__tests__/check-python-path.sh '${{ steps.setup-python.outputs.python-path }}'
+      shell: bash
+
+    - name: Validate version
+      run: ${{ startsWith(steps.setup-python.outputs.python-version, '3.12.') }}
+      shell: bash
+
+    - name: Run simple code
+      run: python -c 'import math; print(math.factorial(5))'
+
   setup-versions-noenv:
     name: Setup ${{ matrix.python }} ${{ matrix.os }} (noenv)
     runs-on: ${{ matrix.os }}
@@ -223,4 +252,4 @@ jobs:
             exit 1
           }
           $pythonVersion
-        shell: pwsh
+        shell: pwsh