Since ebd18f3f (golang_str: bstr/ustr pickle support) bstr and ustr have
support for pickling. However in that patch I verified that it is
possible to dump and load back an object only on the same python
version, which missed that fact that a bstr pickled on py2 cannot be
loaded on py3:

on py2:

    (z-dev) kirr@deca:~/src/tools/go/pygolang$ ipython
    Python 2.7.18 (default, Apr 28 2021, 17:39:59)
    ...

    In [1]: from golang import *

    In [2]: s = bstr('мир') + b'\xff'

    In [3]: s
    Out[3]: b(b'мир\xff')

    In [5]: import pickle

    In [6]: p = pickle.dumps(1)

    In [7]: p
    Out[7]: 'I1\n.'

    In [8]: import pickletools

    In [9]: p = pickle.dumps(s, 1)

    In [10]: p
    Out[10]: 'ccopy_reg\n_reconstructor\nq\x00(cgolang._golang\n_pybstr\nq\x01h\x01U\x07\xd0\xbc\xd0\xb8\xd1\x80\xffq\x02tq\x03Rq\x04.'

    In [11]: pickletools.dis(p)
        0: c    GLOBAL     'copy_reg _reconstructor'
       25: q    BINPUT     0
       27: (    MARK
       28: c        GLOBAL     'golang._golang _pybstr'
       52: q        BINPUT     1
       54: h        BINGET     1
       56: U        SHORT_BINSTRING '\xd0\xbc\xd0\xb8\xd1\x80\xff'
       65: q        BINPUT     2
       67: t        TUPLE      (MARK at 27)
       68: q    BINPUT     3
       70: R    REDUCE
       71: q    BINPUT     4
       73: .    STOP
    highest protocol among opcodes = 1

on py3:

    (py39.venv) kirr@deca:~/src/tools/go/pygolang-master$ ipython
    Python 3.9.19+ (heads/3.9:40d77b93672, Apr 12 2024, 06:40:05)
    ...

    In [1]: from golang import *

    In [2]: import pickle

    In [3]: p = b'ccopy_reg\n_reconstructor\nq\x00(cgolang._golang\n_pybstr\nq\x01h\x01U\x07\xd0\xbc\xd0\xb8\xd1\x80\xffq\x02tq\x03Rq\x04.'

    In [4]: s = pickle.loads(p)
    ---------------------------------------------------------------------------
    UnicodeDecodeError                        Traceback (most recent call last)
    Cell In[4], line 1
    ----> 1 s = pickle.loads(p)

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

which happens in the above example because pickling bstr relies on
SHORT_BINSTRING opcode which is not really handled well on py3.

-> Rework how bstr and ustr are pickled by fully taking control on what
we emit at which protocol level and how and asserting in tests that
pickling produces exactly the data, that is expected to be on the
output.

This way we know that pickling bstr/ustr works the same way on both py2
and py3 and, by also asserting that that data can be unpickled and into
the same string object, that both py2 and py3 can load what any of py2
or py3 saved.

For the reference the dump for above b(b'мир\xff') now becomes

    In [5]: p
    Out[5]: 'cgolang\nbstr\nq\x00(X\t\x00\x00\x00\xd0\xbc\xd0\xb8\xd1\x80\xed\xb3\xbfq\x01tq\x02Rq\x03.'

    In [7]: pickletools.dis(p)
        0: c    GLOBAL     'golang bstr'
       13: q    BINPUT     0
       15: (    MARK
       16: X        BINUNICODE u'\u043c\u0438\u0440\udcff'
       30: q        BINPUT     1
       32: t        TUPLE      (MARK at 15)
       33: q    BINPUT     2
       35: R    REDUCE
       36: q    BINPUT     3
       38: .    STOP
    highest protocol among opcodes = 1

See comments in the code, and added golden vectors in the test for details.