Drop support for Python 3.7

* Drop support for Python 3.7, closes #616
* Bumped versions on a whole lot of GitHub Actions

.github/workflows/spellcheck.yml

@@ -6,12 +6,12 @@ jobs:
   spellcheck:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
-    - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
+    - uses: actions/checkout@v4
+    - name: Set up Python
+      uses: actions/setup-python@v5
       with:
-        python-version: 3.9
-    - uses: actions/cache@v2
+        python-version: "3.12"
+    - uses: actions/cache@v4
       name: Configure pip caching
       with:
         path: ~/.cache/pip