Trying Arrow for next release

[Alessandro Molina <al...@ursacomputing.com>]

The development team is preparing for making a new release of arrow in the
next days/weeks if everything goes well and the release gets approved.

To prevent shipping possible regressions, future problems and allow the
contributors to address any of those before it gets released it would be
great if users who have time would be willing to test the current state of
Arrow and check if they face any regression.

Python and R users can try the current state of arrow at any time following
the instructions at

   * Python:
https://arrow.apache.org/docs/python/install.html#installing-nightly-packages
   * R: https://arrow.apache.org/docs/r/#installing-a-development-version

Please make sure you compare to the current 4.0.1 release, to reduce the
risk of incurring in just deprecations or api changes that are not
regressions.

Thanks to everyone willing to help by verifying this!