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/hgweb/server.py

@@ -344,7 +344,7 b' try:'
     threading.active_count()  # silence pyflakes and bypass demandimport
     _mixin = socketserver.ThreadingMixIn
 except ImportError:
-    if util.safehasattr(os, b"fork"):
+    if util.safehasattr(os, "fork"):
         _mixin = socketserver.ForkingMixIn
     else: