Make message.to_dict() return bytes for DATA type field (#386)

* change to_dict return bytes for DATA type

* reformat

* reformat

* add includes .h

---------

Co-authored-by: Brian Xu <brian.xu1@bytedance.com>

test/test_blob_to_dict_base64.py

@@ -1,5 +1,6 @@
 import os
 import capnp
+import base64
 import pytest
 
 this_dir = os.path.dirname(__file__)
@@ -14,7 +15,7 @@ def test_blob_to_dict(blob_schema):
     blob_value = b"hello world"
     blob = blob_schema.BlobTest(blob=blob_value)
     blob_dict = blob.to_dict(encode_bytes_as_base64=True)
-    assert blob_dict["blob"].tobytes() == blob_value
+    assert base64.b64decode(blob_dict["blob"]) == blob_value
     msg = blob_schema.BlobTest.new_message()
     msg.from_dict(blob_dict)
     assert blob.blob == blob_value