Added Python 3.8

[radarhere]

No description provided.

[radarhere]

The macOS install was failing because it was looking for 'python-3.8.0-macosx10.6.pkg' - which doesn't exist in https://www.python.org/ftp/python/3.8.0/. It looks like since https://www.python.org/ftp/python/3.7.5/, 10.6 has been dropped, leaving only 10.9. I updated the commit, and the builds now pass.

[radarhere]

It may also be worth noting that we are diverging from multibuild master into multibuild devel - but
https://github.com/matthew-brett/multibuild/blob/devel/README.rst

We try to keep the master branch stable and do testing and development in the devel branch. From time to time we merge devel into master.

In practice, you can check out the newest commit from devel that works for you, then stay at it until you need newer features.

[hugovk]

Thanks!

Does that affect the minimum macOS we support?

[radarhere]

The deployment target for our builds is 10.10 - so we should be fine with installing a package for 10.9.

[hugovk]

Thank you!