golang_str: Fix bstr.decode to handle 'string-escape' codec properly

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.

golang/_golang_str.pyx

@@ -422,6 +422,9 @@ class pybstr(bytes):
             x = _utf8_decode_surrogateescape(self)
         else:
             x = bytes.decode(self, encoding, errors)
+        # on py2 e.g. bytes.decode('string-escape') returns bytes
+        if PY_MAJOR_VERSION < 3  and  isinstance(x, bytes):
+            return pyb(x)
         return pyu(x)
 
     if PY_MAJOR_VERSION < 3:

golang/golang_str_test.py

@@ -690,6 +690,17 @@ def test_strings_encodedecode():
     with raises(UnicodeEncodeError):
         u_k8mir.encode('ascii')
 
+    # on py2 there are encodings for which bytes.decode returns bytes
+    # e.g. bytes.decode('string-escape') is actually used by pickle
+    # verify that this exact semantic is preserved
+    if six.PY3:
+        with raises(LookupError):  bs.decode('hex')
+        with raises(LookupError):  bs.decode('string-escape')
+    else:
+        _ = bs.decode('string-escape');          assert type(_) is bstr;  assert _ == bs
+        _ = b(r'x\'y').decode('string-escape');  assert type(_) is bstr;  assert _bdata(_) == b"x'y"
+        _ = b('616263').decode('hex');           assert type(_) is bstr;  assert _bdata(_) == b"abc"
+
 
 # verify string operations like `x * 3` for all cases from bytes, bytearray, unicode, bstr and ustr.
 @mark.parametrize('tx', (bytes, unicode, bytearray, bstr, ustr))