And to convert them to bstr/ustr decoding buffer data as if it was
bytes. This is needed if e.g. we have data in mmap or numpy.ndarray, and
want to convert the data to string. The conversion is always explicit via
explicit call to b/u. And for bstr/ustr constructors, we preserver their
behaviour to match unicode constructor not to convert automatically, but
instead to stringify the object, e.g. as shown below:

    In [1]: bdata = b'hello 123'

    In [2]: mview = memoryview(bdata)

    In [3]: str(mview)
    Out[3]: '<memory at 0x7fb226b26700>'	# NOTE _not_ b'hello 123'