Bump pytest-mock from 3.2.0 to 3.3.1 (#527)

Author: dependabot[bot]

requirements/requirements_dev.txt

@@ -15,6 +15,6 @@ pytest-black==0.3.10
 pytest-cov==2.10.0
 pytest-datadir==1.3.1
 pytest-flake8==1.0.6
-pytest-mock==3.2.0
+pytest-mock==3.3.1
 pytest-pylint==0.17.0
 zest.releaser==6.21.0