### Summary
"In 2016, with the release of macOS 10.12 Sierra, the name was changed from OS X to macOS"
source :: https://en.wikipedia.org/wiki/MacOS#macOS
### Checklist
- [ ] If code changes were made then they have been tested.
- [ ] I have updated the documentation to reflect the changes.
- [ ] This PR fixes an issue.
- [ ] This PR adds something new (e.g. new method or parameters).
- [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
- [X] This PR is **not** a code change (e.g. documentation, README, ...)
Add long_description_content_type in setup.py, fix incorrectly
matched up title underlines in README.rst, and fix incorrect entry in
MANIFEST.in that was forgotten when the README was renamed in 06296bf.
Soheab Malik
Rapptz
Hornwitser
Ben Mintz
Giovani Irimea