Update tox from 3.15.1 to 3.15.2 (#361)

Author: pyup-bot

requirements-testing.txt

@@ -7,7 +7,7 @@ autopep8==1.5.3
 mypy==0.780
 py-spy==0.3.3
 codecov==2.1.4
-tox==3.15.1
+tox==3.15.2
 mccabe==0.6.1
 pylint==2.5.2
 rope==0.17.0