qq is used to quote strings or byte-strings. The following example
illustrates the problem we are currently hitting in zodbtools with
Python3:

    >>> "hello %s" % qq("мир")
    'hello "мир"'

    >>> b"hello %s" % qq("мир")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires a bytes-like object, or an object that implements __bytes__, not 'str'

    >>> "hello %s" % qq(b("мир"))
    'hello "мир"'

    >>> b"hello %s" % qq(b("мир"))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires a bytes-like object, or an object that implements __bytes__, not 'str'

i.e. one way or another if type of format string and what qq returns do not
match it creates a TypeError.

We want qq(obj) to be useable with both string and bytestring format.

For that let's teach qq to return special str- and bytes- derived types that
know how to automatically convert to str->bytes and bytes->str via b/u
correspondingly. This way formatting works whatever types combination it was
for format and for qq, and the whole result has the same type as format.

For now we teach only qq to use new types and don't generally expose
_str and _unicode to be returned by b and u yet. However we might do so
in the future after incrementally gaining a bit more experience.

/proposed-for-review-on: nexedi/pygolang!1