add support for python-version-file (#336)

* add support for python-version-file

* Update action.yml

* update to v4, remove python-version default

* python-version overrides python-version-file, like setup-node
* checks '.python-version' by default if nothing else specified

* update tests, update to checkout@v3

* update build

* appease the linter

* remove old test for default python version

* revert readme changes

* update build

.github/workflows/test-python.yml

@@ -10,33 +10,9 @@ on:
       - '**.md'
   schedule:
     - cron: 30 3 * * *
+  workflow_dispatch:
 
 jobs:
-  default-version:
-    name: Setup default version
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [macos-latest, windows-latest, ubuntu-18.04, ubuntu-20.04]
-    steps:
-    - name: Checkout
-      uses: actions/checkout@v2
-
-    - name: setup default python
-      id: setup-python
-      uses: ./
-
-    - name: Check python-path
-      run: ./__tests__/check-python-path.sh '${{ steps.setup-python.outputs.python-path }}'
-      shell: bash
-
-    - name: Validate version
-      run: python --version
-
-    - name: Run simple python code
-      run: python -c 'import math; print(math.factorial(5))'
-
   setup-versions-from-manifest:
     name: Setup ${{ matrix.python }} ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
@@ -47,7 +23,7 @@ jobs:
         python: [3.5.4, 3.6.7, 3.7.5, 3.8.1]
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
     - name: setup-python ${{ matrix.python }}
       id: setup-python
@@ -72,6 +48,44 @@ jobs:
     - name: Run simple code
       run: python -c 'import math; print(math.factorial(5))'
 
+  setup-versions-from-file:
+    name: Setup ${{ matrix.python }} ${{ matrix.os }} version file
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [macos-latest, windows-latest, ubuntu-18.04, ubuntu-20.04]
+        python: [3.5.4, 3.6.7, 3.7.5, 3.8.1]
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+
+    - name: build-version-file ${{ matrix.python }}
+      run: echo ${{ matrix.python }} > .python-version
+
+    - name: setup-python ${{ matrix.python }}
+      id: setup-python
+      uses: ./
+      with:
+        python-version-file: '.python-version'
+
+    - name: Check python-path
+      run: ./__tests__/check-python-path.sh '${{ steps.setup-python.outputs.python-path }}'
+      shell: bash
+
+    - name: Validate version
+      run: |
+        $pythonVersion = (python --version)
+        if ("Python ${{ matrix.python }}" -ne "$pythonVersion"){
+          Write-Host "The current version is $pythonVersion; expected version is ${{ matrix.python }}"
+          exit 1
+        }
+        $pythonVersion
+      shell: pwsh
+
+    - name: Run simple code
+      run: python -c 'import math; print(math.factorial(5))'
+
   setup-pre-release-version-from-manifest:
     name: Setup 3.9.0-beta.4 ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
@@ -81,7 +95,7 @@ jobs:
         os: [macos-latest, windows-latest, ubuntu-18.04, ubuntu-20.04]
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
     - name: setup-python 3.9.0-beta.4
       id: setup-python