Decoded field tables have byte string keys in Python3

[everilae]

Now, this could be by-design and if so, please close this issue forthwith... but:

Looking at _maybe_utf8 one gets the picture that keys would be decoded to unicode strings, if they are utf-8 byte strings. As it turns out, this is not the case. For example:

>>> pamqp.decode.field_table(pamqp.encode.field_table({'A': 1}))
(9, {b'A': 1})

I think it's not quite far-fetched that this will confuse PY3 users, who have to handle encoding key values at the receiving end, or suffer KeyErrors in order to find out that they must use bytes.

[gmr]

Addressed in 1.7, thanks!