On py2 str.decode('string-escape') returns str, not unicode and this
property is actually being used and relied upon by Lib/pickle.py:

https://github.com/python/cpython/blob/v2.7.18-0-g8d21aa21f2c/Lib/pickle.py#L967-L977

We promised bstr to be drop-in replacement for str on py2, so let's
adjust its behaviour to match the original because if we do not,
unpickling strings will break when str is replaced by bstr under
gpython.

Do not add bstr.encode yet until we hit a real case where it is actually used.