Bump pytest-mock from 3.3.1 to 3.4.0 (#670)

Author: dependabot[bot]

requirements/requirements_dev.txt

@@ -15,5 +15,5 @@ pytest-black==0.3.12
 pytest-cov==2.10.1
 pytest-datadir==1.3.1
 pytest-flake8==1.0.6
-pytest-mock==3.3.1
+pytest-mock==3.4.0
 pytest-pylint==0.17.0