work

.github/actions/python-versions/action.yml

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+name: get test environments
+description: Inspect a Python project and generate a test environment matrix.
+inputs:
+  windows-env:
+    description: Include a Windows environment with the latest Python version.
+    required: false
+    default: 'false'
+  macos-env:
+    description: Include a macOS environment with the latest Python version.
+    required: false
+    default: 'false'
+  extra-envs:
+    description: Add these additional environments to the output.
+    required: false
+    default: ''
+outputs:
+  matrix:
+    description: A JSON list of test environments.
+    value: ${{ steps.run.outputs.matrix }}
+runs:
+  using: composite
+  steps:
+    - name: get test environments
+      id: run
+      shell: bash
+      run: python3 -Iu '${{github.action_path}}/run.py'

.github/actions/python-versions/run.py

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+import configparser
+import os
+import tomllib
+
+tox_conf = configparser.ConfigParser(interpolation=None)
+tox_conf.read("tox.ini", "utf-8")
+
+with open("pyproject.toml", "rb") as f:
+    pyproj = tomllib.load(f)
+
+print(tox_conf)
+print(tomllib)
+
+with open(os.environ["GITHUB_OUTPUT"], "a", encoding="utf-8") as f:
+    print("[]", file=f)