CI: Add Python 3.13 to "nightly" job

Author: amotl

.github/workflows/nightly.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         os: ['ubuntu-latest']
-        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12']
+        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
         cratedb-version: ['nightly']
 
       fail-fast: false