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/commandserver.py

@@ -332,7 +332,7 b' class server:'
             # any kind of interaction must use server channels, but chg may
             # replace channels by fully functional tty files. so nontty is
             # enforced only if cin is a channel.
-            if not util.safehasattr(self.cin, b'fileno'):
+            if not util.safehasattr(self.cin, 'fileno'):
                 ui.setconfig(b'ui', b'nontty', b'true', b'commandserver')
 
         req = dispatch.request(