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ARROW-4098: [Python] Deprecate open_file/open_stream top level APIs in favor of using ipc namespace

This will mean some user code will have to change (e.g. https://github.com/apache/spark/blob/8edae94fa7ec1a1cc2c69e0924da0da85d4aac83/python/pyspark/serializers.py#L240) but it is the most maintainable option for the long term. We should not remove the deprecated APIs until we are confident that at least our open source downstream dependencies are taken care of

Author: Krisztián Szűcs <szucs.krisztian@gmail.com>
Author: Wes McKinney <wesm+git@apache.org>

Closes #3244 from wesm/ARROW-4098 and squashes the following commits:

ec3c54be <Krisztián Szűcs> update ipc doc
9017e7ff <Krisztián Szűcs> remove accidentally committed file
36b6a861 <Wes McKinney> Fix up API docs
7ed5343e <Wes McKinney> Deprecate pyarrow.open_stream/open_file in favor of ipc-namespaced versions