Explicitly drop support for python on Windows.

This was already broken, but wasn't caught in tests because `python3` wasn't defined on our windows machines.  To prevent it from breaking the entire downstream Bazel workspace when `python3` exists, we mark it unsupported.

PiperOrigin-RevId: 506318286
diff --git a/bazel/system_python.bzl b/bazel/system_python.bzl
index 2979c14..5cb34d5 100644
--- a/bazel/system_python.bzl
+++ b/bazel/system_python.bzl
@@ -195,6 +195,10 @@
         if int(python_version[idx]) < int(v):
             supported = False
             break
+    if "win" in ctx.os.name:
+        # buildifier: disable=print
+        print("WARNING: python is not supported on Windows")
+        supported = False
 
     build_file = _build_file.format(
         interpreter = python3,