safehasattr: pass attribute name as string instead of bytes

This is a step toward replacing `util.safehasattr` usage with plain `hasattr`.
The builtin function behave poorly in Python2 but this was fixed in Python3.

These change are done one by one as they tend to have a small odd to trigger
puzzling breackage.

mercurial/dispatch.py

@@ -107,7 +107,7 b' class request:'
 def _flushstdio(ui, err):
     status = None
     # In all cases we try to flush stdio streams.
-    if util.safehasattr(ui, b'fout'):
+    if util.safehasattr(ui, 'fout'):
         assert ui is not None  # help pytype
         assert ui.fout is not None  # help pytype
         try: