Add 18 tests.

Author: dwsteele

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
 
       # Test all supported versions
       matrix:
-        pgver: [13, 14, 15, 16, 17]
+        pgver: [13, 14, 15, 16, 17, 18]
 
     steps:
       - name: Checkout Code