Annotate node_bytes type for mypy 2.0.0

mypy 2.0.0 narrows node_bytes to bytes from the 24-bit branch's
assignment and rejects the bytearray reassignment and mutation in the
28-bit branch. Widen the declared type to cover both.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: horgh

maxminddb/reader.py

@@ -220,6 +220,7 @@ def _read_node(self, node_number: int, index: int) -> int:
         base_offset = node_number * self._metadata.node_byte_size
 
         record_size = self._metadata.record_size
+        node_bytes: bytes | bytearray
         if record_size == 24:
             offset = base_offset + index * 3
             node_bytes = b"\x00" + self._buffer[offset : offset + 3]