Don't run python tests on windows, which aren't supported anyway (#25598)

Author: mkruskal-google

.github/workflows/test_upb.yml

@@ -96,7 +96,7 @@ jobs:
         with:
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
           bazel-cache: "upb-bazel-windows"
-          bazel: test --cxxopt=/std:c++17 --host_cxxopt=/std:c++17 //upb/... //upb_generator/... //python/...
+          bazel: test --cxxopt=/std:c++17 --host_cxxopt=/std:c++17 //upb/... //upb_generator/...
           version: 6.4.0
           exclude-targets: -//python:conformance_test -//upb/reflection:def_builder_test