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

@@ -214,7 +214,7 b' class _httprequesthandler(httpservermod.'
         env['wsgi.multithread'] = isinstance(
             self.server, socketserver.ThreadingMixIn
         )
-        if util.safehasattr(socketserver, b'ForkingMixIn'):
+        if util.safehasattr(socketserver, 'ForkingMixIn'):
             env['wsgi.multiprocess'] = isinstance(
                 self.server, socketserver.ForkingMixIn
             )