Update Python and ANTLR test matrix versions.

Author: hodgestar

.github/workflows/tests-ast.yml

@@ -13,9 +13,18 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # Just using minimum and maximum to avoid exploding the matrix.
-        python-version: ['3.7', '3.12']
-        antlr-version: ['4.7', '4.13']
+        include:
+            # Build on the oldest and latest supported Pythons
+            # and on the oldest and latest ANTLR version supported
+            # by each of those Pythons:
+            - python-version: '3.9'
+              antlr-version: '4.7'
+            - python-version: '3.9'
+              antlr-version: '4.13'
+            - python-version: '3.13'
+              antlr-version: '4.9'
+            - python-version: '3.13'
+              antlr-version: '4.13'
     defaults:
       run:
         working-directory: source/openpulse