Fix staleness tests under non-utf8 bytes

upb/cmake/staleness_test_lib.py

diff --git a/upb/cmake/staleness_test_lib.py b/upb/cmake/staleness_test_lib.py
index d9b78a6..3b32405 100644
--- a/upb/cmake/staleness_test_lib.py
+++ b/upb/cmake/staleness_test_lib.py
@@ -121,7 +121,7 @@
       missing_files.append(pair)
       continue
 
-    with open(pair.generated) as g, open(pair.target) as t:
+    with open(pair.generated, 'rb') as g, open(pair.target, 'rb') as t:
       if g.read() != t.read():
         stale_files.append(pair)