Issue #8313: traceback.format_exception_only() encodes unicode message to
ASCII with backslashreplace error handler if str(value) failed
diff --git a/Lib/traceback.py b/Lib/traceback.py
index 8d28afc..8cb9e28 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -211,8 +211,14 @@
 def _some_str(value):
     try:
         return str(value)
-    except:
-        return '<unprintable %s object>' % type(value).__name__
+    except Exception:
+        pass
+    try:
+        value = unicode(value)
+        return value.encode("ascii", "backslashreplace")
+    except Exception:
+        pass
+    return '<unprintable %s object>' % type(value).__name__
 
 
 def print_exc(limit=None, file=None):