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/bundle2.py

@@ -1381,7 +1381,7 b' class unbundlepart(unpackermixin):'
     def __init__(self, ui, header, fp):
         super(unbundlepart, self).__init__(fp)
         self._seekable = util.safehasattr(fp, 'seek') and util.safehasattr(
-            fp, b'tell'
+            fp, 'tell'
         )
         self.ui = ui
         # unbundle state attr