add missing __all__ to type stubs

[jorenham]

• I wrote this (no AI-generated code or text is included in this PR or its description)
• any code changes are tested, if possible

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According to the Python typing spec:

A stub file should contain an __all__ variable if and only if it is also present at runtime. In that case, the contents of __all__ should be identical in the stub and at runtime. If the runtime dynamically adds or removes elements (for example if certain functions are only available on some system configurations), include all possible elements in the stubs.

I also verified that stubtest now doesn't complain about these __all__ anymore (with stubtest --mypy-config-file=mypy.ini zmq to be precise).

This should also help tools like typestats to index the public API, which at the moment isn't all too accurate: https://jorenham.github.io/typestats/dashboard/report/#pyzmq