Add functions PyUnicode_Append() and PyUnicode_AppendAndDel() that mirror

PyString_Concat() and PyString_ConcatAndDel() (the name PyUnicode_Concat()
was already taken).

Change PyObject_Repr() to always return a unicode object.

Update all repr implementations to return unicode objects.

Add a function PyObject_ReprStr8() that calls PyObject_Repr() and converts
the result to an 8bit string.

Use PyObject_ReprStr8() where using PyObject_Repr() can't be done
straightforward.

Modules/socketmodule.c

@@ -2865,7 +2865,7 @@ sock_repr(PySocketSockObject *s)
 		(long)s->sock_fd, s->sock_family,
 		s->sock_type,
 		s->sock_proto);
-	return PyString_FromString(buf);
+	return PyUnicode_FromString(buf);
 }