bytearray was introduced in Python as a mutable version of bytes. It has
all strings methods (e.g. .capitalize() .islower(), etc), and it also
supports % formatting. In other words it has all attributes of being a
byte-string, with the only difference from bytes in that bytearray is
mutable. In other words bytearray is handy to have when a string is
being incrementally constructed step by step without hitting overhead of
many bytes objects creation/destruction.

So, since bytearray is also a bytestring, similarly to bytes, let's add
support to interoperate with bytearray to bstr and ustr:

- b/u and bstr/ustr now accept bytearray as argument and treat it as bytestring.
- bytearray() constructor, similarly to bytes() and unicode()
  constructors, now also accepts bstr/ustr and create bytearray object
  corresponding to byte-stream of input.

For the latter point to work we need to patch bytearray.__init__() a bit,
since, contrary to bytes.__init__(), it does not pay attention to
whether provided argument has __bytes__ method or not.