Bump pytest-black from 0.3.7 to 0.3.8 (#268)

Author: dependabot-preview[bot]

requirements/requirements_dev.txt

@@ -11,7 +11,7 @@ placebo==0.9.0
 pre-commit==2.0.1
 pylint==2.4.4
 pytest==5.3.5
-pytest-black==0.3.7
+pytest-black==0.3.8
 pytest-cov==2.8.1
 pytest-datadir==1.3.1
 pytest-flake8==1.0.4