fix: GitHub Action can now be used on ubuntu-18.04 runner (#606)

* chore: add tests for the GitHub Action

* fix: GitHub Action can be used on ubuntu-18.04

Always install nox with python3.10 in order to allow
using the action with an ubuntu-18.04 runner.

.github/workflows/action.yml

@@ -0,0 +1,22 @@
+name: Action
+on:
+  push:
+    branches:
+      - 'main'
+      - '**action**'
+  pull_request:
+    paths:
+      - '.github/workflows/action.yml'
+      - 'action.yml'
+env:
+  FORCE_COLOR: "1"
+jobs:
+  action-default-tests:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-18.04, ubuntu-20.04, ubuntu-22.04, windows-2019, windows-2022, macos-10.15, macos-11, macos-12]
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ./
+      - run: nox --non-interactive --error-on-missing-interpreter --session github_actions_default_tests

action.yml

@@ -31,5 +31,6 @@ runs:
         python-version: "3.10"
 
     - name: "Install nox"
-      run: pipx install '${{ github.action_path }}'
+      # --python "$(which python)" => always use the last setup-python version to install nox.
+      run: pipx install --python "$(which python)" '${{ github.action_path }}'
       shell: bash

noxfile.py

@@ -105,3 +105,31 @@ def docs(session: nox.Session) -> None:
         sphinx_args.insert(0, "--open-browser")
 
     session.run(sphinx_cmd, *sphinx_args)
+
+
+# The following sessions are only to be run in CI to check the nox GHA action
+def _check_python_version(session: nox.Session) -> None:
+    if session.python.startswith("pypy"):
+        python_version = session.python[4:]
+        implementation = "pypy"
+    else:
+        python_version = session.python
+        implementation = "cpython"
+    session.run(
+        "python",
+        "-c",
+        "import sys; assert '.'.join(str(v) for v in sys.version_info[:2]) =="
+        f" '{python_version}'",
+    )
+    if python_version[:2] != "2.":
+        session.run(
+            "python",
+            "-c",
+            f"import sys; assert sys.implementation.name == '{implementation}'",
+        )
+
+
+@nox.session(python=["3.7", "3.8", "3.9", "3.10", "pypy3.7", "pypy3.8", "pypy3.9"])
+def github_actions_default_tests(session: nox.Session) -> None:
+    """Check default versions installed by the nox GHA Action"""
+    _check_python_version(session)