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

@@ -331,7 +331,7 b' def _runcatch(req):'
 
         ui = req.ui
         try:
-            for name in b'SIGBREAK', b'SIGHUP', b'SIGTERM':
+            for name in 'SIGBREAK', 'SIGHUP', 'SIGTERM':
                 num = getattr(signal, name, None)
                 if num:
                     signal.signal(num, catchterm)