Rename OSX to Darwin/macOS

[JMY1000]

OSX is no longer the appropriate term for the OS. Things should be renamed to Darwin and macOS as appropriate.

^{Not exactly a high priority issue, but I thought it'd be worth putting into writing.}

[giampaolo]

Mmm yeah. There are two things involved here. The "OSX" occurrences in code comments, docstrings and doc (and that's easy to change) and the psutil.OSX constant which is more debatable as it's a public API.
We either add psutil.MACOS and deprecate psutil.OSX or don't do anything. Maybe I'm a little bit more keen on not doing anything.

[giampaolo]

Note: https://www.apple.com/osx redirects to https://www.apple.com/macos/high-sierra/ so they're pretty serious about it.

[giampaolo]

OK, I changed my mind and decided to rename everything (including source files) in PR #1298 which introduces psutil.MACOS and deprecates psutil.OSX. If "macOS" really is the future I suppose "OSX" will gradually be forgotten. Even if it's gonna take years maybe it makes more sense to just be done with it immediately.