Update tox from 3.21.1 to 3.21.3 (#493)

Author: pyup-bot

requirements-testing.txt

@@ -7,7 +7,7 @@ autopep8==1.5.4
 mypy==0.790
 py-spy==0.3.3
 codecov==2.1.11
-tox==3.21.1
+tox==3.21.3
 mccabe==0.6.1
 pylint==2.6.0
 rope==0.18.0