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Co-authored-by: Andrew Murray <3112309+radarhere@users.noreply.github.com>

MRG: Added IS_MACOS

Apple left 'OS X' behind with 10.11, and now we've moved on to 11.

So multibuild's variable `IS_OSX` is not quite as suitable as it once was.

This PR adds `IS_MACOS` - but still sets `IS_OSX` to not break any external scripts that may be using it.

Updated Python versions

Updated Travis job to a macOS supported by Homebrew

MRG: Updated Python versions

Updated arb to 2.19.0

Fixed typo

Updated README

Added Python 3.9

Updated AppVeyor rc jobs to Python 3.9

Updated Python versions

Update openblas download from anaconda

MRG: Update python 2.7 to 2.7.18

Very minor changes over 2.7.17, but a couple are worth having for anyone still using 2.7
see https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.18rc1.rst

MRG: Install cmake instead if cmake28 cannot be found in yum

When attempting to build on aarch64, I find that cmake28 is not found. However, cmake does exist.

So this PR adds `get_modern_cmake` that searches for cmake28, and if it is not found, then use cmake instead.

Co-authored-by: Andrew Murray <3112309+radarhere@users.noreply.github.com>

MRG: document switch from Rackspace

[skip ci]

Thanks @hugovk
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

Thanks to @mattip

Updated Python versions