Bump pytest-cov from 2.12.1 to 3.0.0 (#1162)

Author: dependabot[bot]

requirements/requirements_dev.txt

@@ -13,7 +13,7 @@ pre-commit==2.13.0
 pylint==2.7.4
 pytest==6.2.5
 pytest-black==0.3.12
-pytest-cov==2.12.1
+pytest-cov==3.0.0
 pytest-datadir==1.3.1
 pytest-flake8==1.0.7
 pytest-mock==3.6.1