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ARROW-3641: [Python] Remove unneeded public keyword from pyarrow public C APIs

According to

https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#c-api-declarations

it is not necessary to use `public` here. If we want to be able to refer to Cython extension types at the C API level (at some point, this may not be a bad idea), then we must use `public` with those.

Author: Wes McKinney <wesm+git@apache.org>

Closes #3147 from wesm/ARROW-3641 and squashes the following commits:

f09902cb4 <Wes McKinney> Remove unneeded public keyword from pyarrow public APIs