Update name to macOS

[hugovk]

Apple's operating system has been called macOS since 2016. Before that it was OS X, and before that it was Mac OS X.

In most cases we're referring to the recent versions, so let's use the current name.

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We have one section specifically with two sets of instructions:

• older set: Mac OS X (versions 10.0-10.7) and OS X 10.8
• newer set: OS X 10.9-10.11 and macOS (versions 10.12+)

It's good to differentiate the names here. But whilst we're reviewing this, are the instructions for the older set still useful?

• Mac OS X versions 10.0-10.7 were released between 2001-2012
• OS X 10.8 was released between 2012-2015

Similarly, do we still need to mention OS X 10.9+ in the newer set?

• OS X versions 10.9-10.11 were released between 2013-2018

Mac OS X and OS X are all long out of support.

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Finally, I noticed the link from https://devguide.python.org/#key-resources to the daily installer goes to https://buildbot.python.org/daily-dmg/, but it's an empty directory listing.

Should there be an installer there or shall we remove the link?

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cc @ronaldoussoren @ned-deily

[hugovk]

We have one section specifically with two sets of instructions:

• older set: Mac OS X (versions 10.0-10.7) and OS X 10.8
• newer set: OS X 10.9-10.11 and macOS (versions 10.12+)

It's good to differentiate the names here. But whilst we're reviewing this, are the instructions for the older set still useful?

• Mac OS X versions 10.0-10.7 were released between 2001-2012
• OS X 10.8 was released between 2012-2015

Similarly, do we still need to mention OS X 10.9+ in the newer set?

• OS X versions 10.9-10.11 were released between 2013-2018

Mac OS X and OS X are all long out of support.

Please see PR #1006 to remove these old bits.