callRemote() requires a native string, making Python 3 porting harder

[itamarst]

Tahoe-LAFS is being ported to run on both Python 2 and 3 at same time; eventually the former will be dropped, but for now it's both.

We're adding from __future__ import unicode, but that breaks things like remoteref.callRemote("mymethod", arg1) because on Python 2, "mymethod" is now Unicode, but on Python 2 Foolscap expects it to be a native string, i.e. bytes.

It would be good to accept unicode strings on Python 2 as well; I will submit PR that does that.

[itamarst]

Specifically, the issue is

foolscap/src/foolscap/referenceable.py

Line 561 in 845bea5

assert type(name) is str