Bump mock from 3.0.5 to 4.0.2

Bumps [mock](https://github.com/testing-cabal/mock) from 3.0.5 to 4.0.2.
- [Release notes](https://github.com/testing-cabal/mock/releases)
- [Changelog](https://github.com/testing-cabal/mock/blob/master/CHANGELOG.rst)
- [Commits](testing-cabal/mock@3.0.5...4.0.2)

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

requirements-test.txt

@@ -7,4 +7,4 @@ mysqlclient==1.4.6 # rq.filter: !=1.4.2.post1
 port-for==0.4
 pytest-cov==2.7.1
 pytest-xdist==1.29.0
-mock==3.0.5
+mock==4.0.2