main.yml: Save CI resources + document why each version is covered

.github/workflows/main.yml

@@ -13,7 +13,16 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "pypy-3.8", "pypy-3.9", "pypy-3.10"]
+        # NOTE: The idea here is:
+        #       - Cover both CPython and PyPy3
+        #       - Cover both the oldest and most recent version supported
+        #       - Cover switch "<3.10" on both sides at the edge, i.e. cover
+        #         both 3.9 (for "<"/true) and 3.10 (for ">="/false)
+        #         since we have a switch like that
+        #         (see `install_requires` in `setup.py` for details)
+        #       That allows us to save resources on:
+        #       - "3.11"
+        python-version: ["3.8", "3.9", "3.10", "3.12", "pypy-3.8", "pypy-3.9", "pypy-3.10"]
 
     steps:
       - uses: actions/checkout@v4