Bump mock from 4.0.2 to 4.0.3 (#387)

Bumps [mock](https://github.com/testing-cabal/mock) from 4.0.2 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@4.0.2...4.0.3)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

dev-requirements.txt

@@ -3,7 +3,7 @@
 ################
 
 # Mocks (stdlib ones change between versions of Python, so are not consistent in behaviour like the backport is).
-mock==4.0.2
+mock==4.0.3
 
 # Py.test stuff.
 pytest==6.1.2