GHA: Test on Python 3.16 alpha 1

Author: vstinner

.github/workflows/build.yml

@@ -31,6 +31,8 @@ jobs:
           - "3.14t"
           - "3.15"
           - "3.15t"
+          - "3.16"
+          - "3.16t"
 
           # PyPy versions:
           # - https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md