Fix crash when string contains SO but not SI (#489)

The DEC_TAG cs type should not be escaped when a lone SO is found
and should be used in the same way as when both SO and S1 are
present.
Fixes #322

urwid/util.py

@@ -158,7 +158,7 @@ def apply_target_encoding(s: str | bytes):
  sin = sl[0]
  assert isinstance(sin, bytes)
  sout.append(sin)
- rle_append_modify(cout, (escape.DEC_TAG.encode('ascii'), len(sin)))
+ rle_append_modify(cout, (escape.DEC_TAG, len(sin)))
  continue
  sin, son = sl
  son = son.replace(SI, b'')