Fix memory leak in Raw.copy().

Raw_New borrows a reference to its PyBytesObject argument, so Raw_Copy must
still dispose of it.

The following is a minimal repro for this issue:

import msgspec

DECODER = msgspec.msgpack.Decoder(type=msgspec.Raw)
PAYLOAD = b"\xda\xff\xff" + (65535 * b"\0") # array of 65535 zeros

for _ in range(1000000):
  raw = DECODER.decode(PAYLOAD)
  raw.copy()

msgspec/_core.c

@@ -1513,7 +1513,9 @@ Raw_copy(Raw *self, PyObject *unused)
  }
  PyObject *buf = PyBytes_FromStringAndSize(self->buf, self->len);
  if (buf == NULL) return NULL;
- return Raw_New(buf);
+ PyObject *out = Raw_New(buf);
+ Py_DECREF(buf);
+ return out;
 }
 
 static PyMethodDef Raw_methods[] = {