Migrate windows upb tests to bzlmod.

This requires dropping the python tests, which were never actually being run due to system_python's silent failure mode on windows.  These tests could actually be enabled with some fixes in rules_python

PiperOrigin-RevId: 811863667
diff --git a/.github/workflows/test_upb.yml b/.github/workflows/test_upb.yml
index 2fb4174..cf1873a 100644
--- a/.github/workflows/test_upb.yml
+++ b/.github/workflows/test_upb.yml
@@ -91,7 +91,8 @@
         with:
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
           bazel-cache: "upb-bazel-windows"
-          bazel: test //upb/... //upb_generator/... //python/... --noenable_bzlmod
+          # TODO: Enable python tests here once rules_python supports Windows better.
+          bazel: test //upb/... //upb_generator/...
           version: 7.6.1
           exclude-targets: -//python:conformance_test -//upb/reflection:def_builder_test