Bump mock from 2.0.0 to 4.0.3

Bumps [mock](https://github.com/testing-cabal/mock) from 2.0.0 to 4.0.3.
- [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...4.0.3)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -18,7 +18,7 @@ lazy-object-proxy==1.3.1
 Markdown==3.2.2
 mccabe==0.6.1
 mdv==1.7.4
-mock==2.0.0
+mock==4.0.3
 pbr==4.0.2
 pycodestyle==2.4.0
 Pygments==2.2.0