Attempt to pin Python versions to classifiers

Thanks to some magic from Hynek

.github/workflows/tests.yml

@@ -5,6 +5,16 @@ on:
   - pull_request
 
 jobs:
+  build-package:
+    name: Build & inspect our package.
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: hynek/build-and-inspect-python-package@v2
+        id: baipp
+    outputs:
+      python-versions: ${{ steps.baipp.outputs.supported_python_classifiers_json_array }}
   tests:
     name: ${{ matrix.session }} ${{ matrix.python }} / ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
@@ -32,7 +42,7 @@ jobs:
       - name: Set up Python ${{ matrix.python }}
         uses: actions/[email protected]
         with:
-          python-version: ${{ matrix.python }}
+          python-version: ${{ fromJson(needs.check-package.outputs.python-versions) }}
 
       - name: Upgrade pip
         run: |

noxfile.py

@@ -5,6 +5,8 @@
 from textwrap import dedent
 
 import nox
+import tomllib
+
 
 try:
     from nox_poetry import session
@@ -19,7 +21,12 @@
 
 
 package = "countdown"
-python_versions = ["3.10", "3.9", "3.8"]
+config = tomllib.loads(Path("pyproject.toml").read_text())
+python_versions = [
+    classifier.split()[-1]
+    for classifier in config["tool"]["poetry"]["classifiers"]
+    if classifier.startswith("Programming Language :: Python :: ")
+]
 nox.needs_version = ">= 2021.6.6"
 nox.options.sessions = ("pre-commit", "tests")
 

pyproject.toml

@@ -13,6 +13,11 @@ packages = [
 ]
 classifiers = [
     "Development Status :: 4 - Beta",
+    "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: 3.9",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
 ]
 
 [tool.poetry.urls]