Bump mock from 4.0.1 to 4.0.3

Bumps [mock](https://github.com/testing-cabal/mock) from 4.0.1 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@4.0.1...4.0.3)

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

requirements_dev.txt

@@ -4,7 +4,7 @@ black==19.10b0
 coverage==5.0.3
 cryptography==2.8
 flask==1.1.1
-mock==4.0.1
+mock==4.0.3
 pip==20.0.2
 plumbum==1.6.8
 pytest==5.3.5