Bump mock from 2.0.0 to 3.0.5

Bumps [mock](https://github.com/testing-cabal/mock) from 2.0.0 to 3.0.5.
- [Release notes](https://github.com/testing-cabal/mock/releases)
- [Changelog](https://github.com/testing-cabal/mock/blob/master/CHANGELOG.rst)
- [Commits](testing-cabal/mock@2.0.0...3.0.5)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements_dev.txt

@@ -2,5 +2,5 @@ black
 coverage==4.5.2
 coveralls==1.5.1
 flake8==3.6.0
-mock==2.0.0
+mock==3.0.5
 nose==1.3.7