Both bstr and ustr constructors mimic constructor of unicode(= str on py3) -
an object is either stringified, or decoded if it provides buffer
interface, or the constructor is invoked with optional encoding and
errors argument:

    # py2
    class unicode(basestring)
     |  unicode(object='') -> unicode object
     |  unicode(string[, encoding[, errors]]) -> unicode object

    # py3
    class str(object)
     |  str(object='') -> str
     |  str(bytes_or_buffer[, encoding[, errors]]) -> str

Stringification of all bstr/ustr / unicode/bytes is handled
automatically with the meaning to convert to created type via b or u.

We follow unicode semantic for both ustr _and_ bstr, because bstr/ustr
are intended to be used as strings.