Update tox from 3.20.1 to 3.21.0 (#480)

pyup-bot · abhinavsingh · web-flow · commit d1681fc11cca · 2021-01-10T22:13:29.000+05:30
Co-authored-by: Abhinav Singh &lt;mailsforabhinav@gmail.com&gt;
diff --git a/requirements-testing.txt b/requirements-testing.txt
@@ -7,7 +7,7 @@ autopep8==1.5.4
 mypy==0.790
 py-spy==0.3.3
 codecov==2.1.11
-tox==3.20.1
+tox==3.21.0
 mccabe==0.6.1
 pylint==2.6.0
 rope==0.18.0
