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ARROW-9030: [Python] Remove pyarrow/compat.py, move some oft-used utility functions to pyarrow.lib

This is some routine scrubbing that deals with some remnants of the prior dual py2/3 codebase.

Closes #7421 from wesm/ARROW-9030

Authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>