Update mock requirement from ~=3.0 to ~=4.0 (#502)

Updates the requirements on [mock](https://github.com/testing-cabal/mock) to permit the latest version.
- [Release notes](https://github.com/testing-cabal/mock/releases)
- [Changelog](https://github.com/testing-cabal/mock/blob/master/CHANGELOG.rst)
- [Commits](testing-cabal/mock@3.0.0...4.0.0)

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

Author: dependabot-preview[bot]

Pipfile

@@ -19,7 +19,7 @@ tox-travis = "~=0.12"
 
 httpretty = "~=0.9"
 python-dateutil = "~=2.8"
-mock = "~=3.0"
+mock = "~=4.0"
 
 pylint = "~=2.4"
 astroid = "~=2.3"