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

@@ -116,7 +116,7 b' def _flushstdio(ui, err):'
             err = e
             status = -1
 
-    if util.safehasattr(ui, b'ferr'):
+    if util.safehasattr(ui, 'ferr'):
         assert ui is not None  # help pytype
         assert ui.ferr is not None  # help pytype
         try: