ARROW-6053: [Python] Fix pyarrow's RecordBatchStreamReader::Open2 type signature
ARROW-6053: [Python] Fix pyarrow's RecordBatchStreamReader::Open2 type signature

Fixes a type mismatch between Arrow C++'s RecordBatchStreamReader::Open method:

https://github.com/apache/arrow/blob/38b01764da445ce6383b60a50d1e9b313857a3d7/cpp/src/arrow/ipc/reader.h#L66-L67

and its Cython defintion:
https://github.com/apache/arrow/blob/38b01764da445ce6383b60a50d1e9b313857a3d7/python/pyarrow/includes/libarrow.pxd#L950-L952

I don't know if this causes problems in python, but I ran into it today while working with  pyarrow in cython. Closes https://issues.apache.org/jira/browse/ARROW-6053.

Closes #4957 from trxcllnt/fix/pyarrow-rbr-open-signature and squashes the following commits:

f9affdfa1 <ptaylor> fix pyarrow cython RecordBatchStreamReader::Open signature

Authored-by: ptaylor <9b5e68c334274b572dadf4941c134a2f08be9921@me.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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