Bump pytest-mock from 3.3.1 to 3.4.0 (#370)

Bumps [pytest-mock](https://github.com/pytest-dev/pytest-mock) from 3.3.1 to 3.4.0.
- [Release notes](https://github.com/pytest-dev/pytest-mock/releases)
- [Changelog](https://github.com/pytest-dev/pytest-mock/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest-mock/compare/v3.3.1...v3.4.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

requirements-dev.txt

@@ -3,7 +3,7 @@ flake8==3.8.4
 async-timeout==3.0.1
 pytest==6.2.1
 pytest-cov==2.10.1
-pytest-mock==3.3.1
+pytest-mock==3.4.0
 coverage==5.3.1
 sphinx==3.4.0
 pep257==0.7.0