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](https://github.com/testing-cabal/mock/compare/2.0.0...4.0.3)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-tests.txt

@@ -2,6 +2,6 @@
 attrs==20.3.0
 pytest==6.2.1
 pytest-cov==2.10.1
-mock==2.0.0
+mock==4.0.3
 flake8==3.8.4
 pyflakes==2.2.0