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

dependabot-preview[bot] · web-flow · commit d676eed8979c · 2020-02-06T00:18:25.000+09:00
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>
diff --git a/Pipfile b/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"
