golang_str: Make bytes(bstr) -> bstr, unicode(ustr) -> ustr

In other words casting to bytes/unicode preserves pygolang string to
remain pygolang string.

Without the changes to bstr/ustr added test fails as e.g.

    >       assert bytes  (bs) is bs
    E       AssertionError: assert b'\xd0\xbc\xd0\xb8\xd1\x80' is b'\xd0\xbc\xd0\xb8\xd1\x80'
    E        +  where b'\xd0\xbc\xd0\xb8\xd1\x80' = bytes(b'\xd0\xbc\xd0\xb8\xd1\x80')

in other words bytes(bstr) was creating a copy and changing type to bytes.

golang/_golang_str.pyx

@@ -120,7 +120,7 @@ class pybstr(bytes):
     __slots__ = ()
 
 
-    # __bytes__ - no need
+    def __bytes__(self):    return self
     def __unicode__(self):  return pyu(self)
 
     def __str__(self):
@@ -138,7 +138,7 @@ cdef class pyustr(unicode):
     """
 
     def __bytes__(self):    return pyb(self)
-    # __unicode__ - no need
+    def __unicode__(self):  return self
 
     def __str__(self):
         if PY_MAJOR_VERSION >= 3:

golang/golang_str_test.py

@@ -142,6 +142,10 @@ def test_strings_basic():
     assert b(bs) is bs
     assert u(us) is us
 
+    # bytes(b(·)) = identity,   unicode(u(·)) = identity
+    assert bytes  (bs) is bs
+    assert unicode(us) is us
+
     # unicode(b) -> u,  bytes(u) -> b
     _ = unicode(bs);  assert type(_) is ustr
     _ = bytes  (us);  assert type(_) is bstr