- we can use BINSTRING* only if protocol >= 1;
- at protocol 0 we thus have to use text STRING;
- if protocol >= 3 we have to emit the string as unicode pickle object
  the same way as Python3 does. If we don't do - Python3 won't be
  generally able to load our pickle:

	In [1]: s = b'U\x06\xd0\xbc\xd0\xb8\xd1\x80q\x00.'

  	In [2]: from pickletools import dis

  	In [3]: dis(s)
  	    0: U    SHORT_BINSTRING 'миÑ\x80'
  	    8: q    BINPUT     0
  	   10: .    STOP
  	highest protocol among opcodes = 1

  	In [4]: import pickle

  	In [5]: pickle.loads(s)
  	---------------------------------------------------------------------------
  	UnicodeDecodeError                        Traceback (most recent call last)
  	<ipython-input-5-764e4625bc41> in <module>()
  	----> 1 pickle.loads(s)

  	UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)

We already decode unicode pickle objects into string, this way
decode(encode(string)) remains always idempotent.