Update tox from 3.16.0 to 3.16.1 (#386)

Author: pyup-bot

requirements-testing.txt

@@ -7,7 +7,7 @@ autopep8==1.5.3
 mypy==0.782
 py-spy==0.3.3
 codecov==2.1.7
-tox==3.16.0
+tox==3.16.1
 mccabe==0.6.1
 pylint==2.5.3
 rope==0.17.0