Update pytest from 5.4.2 to 6.2.1

requirements_dev.txt

@@ -2,7 +2,7 @@ flake8==3.8.1
 Sphinx==3.0.3
 sphinx_rtd_theme==0.4.3
 tox==3.15.0
-pytest==5.4.2
+pytest==6.2.1
 pytest-runner==5.2
 coverage==4.5.3
 pytest-cov==2.8.1