[TIP] mock 3.0.0 released

[Chris Withers]

Hi All,

I'm pleased to announce the release of mock 3.0.0:
https://pypi.org/project/mock/

This brings to rolling backport up to date with cpython master.

It's been a few years since the last release, so I'd be surprised if 
there weren't some problems.
If you hit any issues, please pin to mock<3 and then:

- If your issue relates to mock functionality, please report in the 
python tracker: https://bugs.python.org

- If your issue is specific to the backport, please report here: 
https://github.com/testing-cabal/mock/issues

If you're unsure, go for the second one and we'll figure it out.

cheers,

Chris