Bump mock from 2.0.0 to 3.0.3

Bumps [mock](https://github.com/testing-cabal/mock) from 2.0.0 to 3.0.3.
- [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...3.0.3)

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

Author: dependabot-support

requirements.txt

@@ -1,6 +1,6 @@
 async-generator==1.10; python_version>="3.6"
 codacy-coverage==1.3.11
-mock==2.0.0
+mock==3.0.3
 pytest-cov==2.6.1
 pytest-tornasync; python_version >= '3.5'
 pytest-trio==0.5.2; python_version>="3.6"