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ARROW-1661: [Python] Build Python 3.7 in manylinux container

All dependencies have now released Python 3.7 compatible version, so use them.

For conda-forge, this might take a bit more time but once they have rebuilt all packages, we can automatically provide a 3.7 package there. See also https://github.com/conda-forge/conda-forge-enhancement-proposals/pull/10

Author: Korn, Uwe <Uwe.Korn@blue-yonder.com>

Closes #2462 from xhochy/ARROW-1661 and squashes the following commits:

c4dead99 <Korn, Uwe> Explain numpy version requirement
907794d0 <Korn, Uwe> ARROW-1661:  Build Python 3.7 in manylinux container