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/wireprotov1server.py

@@ -665,7 +665,7 b' def unbundle(repo, proto, heads):'
                 r = exchange.unbundle(
                     repo, gen, their_heads, b'serve', proto.client()
                 )
-                if util.safehasattr(r, b'addpart'):
+                if util.safehasattr(r, 'addpart'):
                     # The return looks streamable, we are in the bundle2 case
                     # and should return a stream.
                     return wireprototypes.streamreslegacy(gen=r.getchunks())