Prefer https://*.readthedocs.io over http://*.rtfd.org

[jdufresne]

• Requests URL using https instead of http
• Avoids unnecessary redirect of *.rtfd.org -> *.readthedocs.io

*.rtfd.org exists as a means for pasting short URLs, which doesn't much apply for links in documentation.

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs:

• Add a new news fragment into the changelog folder
  • name it $issue_id.$type for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of removal, feature, bugfix, vendor, doc or trivial
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."
• Target: for bugfix, vendor, doc or trivial fixes, target master; for removals or features target features;
• Make sure to include reasonable tests for your change if necessary

Unless your change is a trivial or a documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS, in alphabetical order;

[nicoddemus]

LGTM, thanks!

[coveralls]

Coverage remained the same at 92.565% when pulling f3c666d on jdufresne:readthedocs into bc66f7e on pytest-dev:master.