add __all__

[lucasew]

add all for the main module so wildcard imports
only bring the relevant stuff

Signed-off-by: lucasew lucas59356@gmail.com

[DiagonalArg]

Hi @lucasew / I've been working on keeping the Radicale-Decsync plugin alive (Python 3.12 & removing setuptools/pkg_resources), which necessitated making some modifications to the code here.

I looked at this PR, and it's kind of out of scope for me just trying to keep the project alive; but FYI, AI tells me there appears to be a memory management risk: "In ctypes, passing a Python object to C as 'user data' doesn't automatically increment the Python reference count. This means the Python Garbage Collector could delete the object while the C library still holds the pointer, potentially causing a segfault when get_user_data is called." I just figured I'd like you know in case you wanted to update it.