CI: add CI test on python 3.13 (#520)

Author: theroggy

.github/workflows/tests-conda.yml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-2019]
-        python: ["3.10", "3.11", "3.12"]
+        python: ["3.10", "3.11", "3.12", "3.13"]
         env: ["latest"]
         include:
           # environment with lower versions of optional dependencies