Make Travis CI (and Windows x64 ?) mandatory

Azure Pipelines CI was made optional by @brettcannon: https://bugs.python.org/issue39837

Problem: now, there is no more mandatory CI on Python pull requests. It became possible to merge a PR even if all CIs fail.

I would prefer to also have the Windows 64-bit CI (GitHub Action) mandatory, but I'm not sure if it's possible to only make a single GitHub Action mandatory?