Automate python version matrix on gitub actions

pyproject.toml is the source of truth for supported Python versions. No other files need to be kept in sync.

.github/workflows/tests.yml

@@ -9,13 +9,29 @@ on:
   pull_request:
 
 jobs:
-  build:
+  configure:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Read Python versions from pyproject.toml
+      id: read-versions
+      # produces output like: python_versions=[ "3.9", "3.10", "3.11", "3.12" ]
+      run: >-
+        python_versions=$(
+        grep -oP '(?<=Language :: Python :: )\d\.\d+' pyproject.toml
+        | jq --raw-input .
+        | jq --slurp .
+        ) >> $GITHUB_OUTPUT
+    outputs:
+      python_versions: ${{ steps.read-versions.outputs.python_versions }}
 
+  build:
+    needs: [configure]
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ${{ fromJSON(needs.configure.outputs.python_versions) }}
 
     steps:
     - uses: actions/checkout@v2

pyproject.toml

@@ -9,11 +9,15 @@ requires-python = ">= 3.9"
 classifiers = [
     "Programming Language :: Python",
     "Programming Language :: Python :: 3",
+
+    # The following classifiers are parsed by Github Actions workflows.
+    # Precise formatting is important (no extra spaces, etc.)
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
+
     "License :: OSI Approved :: BSD License",
 ]