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

[BrianXu0623]

This PR is for resolving the issue Regression in 2.2.0: yaml.dump can no longer save capnp messages with Data type fields #385

And also trying to resolve the issue PyPI 2.2.0 source distribution missing PyCustomMessageBuilder.h #384

Background

Cap’n Proto’s C++ layer exposes Data fields as pointers into the message’s memory. Returning bytes forces an immediate copy of that buffer. To avoid that expensive copy (which is a real performance and memory bottleneck for large messages), I made it return a memoryview instead of bytes.

However, memoryview is subject to the limitation that they cannot be pickled, but in C++ this is unnecessary.

This can be resolved by modifying the message.to_dict() method, without needing change to user's code.

Please note that the changes in the test case are simply a rollback, not the introduction of any new modifications.

For the manylinux2014 issue, I added the py_custom_message_builder.h to the binary output. Hopefully it can resolve the issue, but I cannot find a way to validate it.