I'm not sure how much the checklist makes sense for this change, given that there are no external or internal API changes.

This PR changes the _Nothing class to be a singleton. This allows a shorter class, as well as more reliance on inherited object code. Because they are all the same instance _Nothing() is _Nothing() will be True, and thus _Nothing() == _Nothing() will be True without intervention. This will result in a slight performance boost.

I have confirmed that this breaks no tests on my local machine under both 2.7 and 3.6

This is my first PR for the project, so I apologize in advance if there are any mistakes.

Pull Request Check List

This is just a reminder about the most common mistakes. Please make sure that you tick all appropriate boxes. But please read our contribution guide at least once, it will save you unnecessary review cycles!

• Added tests for changed code
• New features have been added to our Hypothesis testing strategy.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file tests/typing_example.py.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• Documentation in .rst files is written using semantic newlines.
• Changes (and possible deprecations) have news fragments in changelog.d.

If you have any questions to any of the points above, just submit and ask! This checklist is here to help you, not to deter you from contributing!