ci: update deprecated actions and test claimed Python 3.9 support

actions/checkout@v2 and actions/setup-python@v1 run on Node.js
versions that GitHub has deprecated. Bump all checkout/setup-python
steps to their current majors (v4/v5).

pyproject.toml declares requires-python >=3.9 (and carries the
importlib_metadata backport for it), but CI only tested 3.10-3.14.
Add 3.9 to the matrix so the claimed support is actually exercised.

Author: ThVerg

.github/workflows/ci.yml

@@ -18,10 +18,10 @@ jobs:
     steps:
 
     - name: 🧰 Checkout
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: 🐍 Setup Python ${{ matrix.pyver }}
-      uses: actions/setup-python@v3
+      uses: actions/setup-python@v5
       with:
         python-version: '3.10'
 
@@ -37,6 +37,7 @@ jobs:
         #- { icon: 🍎, name: macos }
         #- { icon: 🧊, name: windows }
         pyver:
+        - '3.9'
         - '3.10'
         - '3.11'
         - '3.12'
@@ -47,10 +48,10 @@ jobs:
     steps:
 
     - name: 🧰 Repository Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: 🐍 Set up Python ${{ matrix.pyver }}
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.pyver }}