Bump zest-releaser from 6.20.1 to 6.21.0 (#461)

dependabot[bot] · web-flow · commit 0181ab9979d7 · 2020-07-28T23:39:18.000+02:00
diff --git a/requirements/requirements_dev.txt b/requirements/requirements_dev.txt
@@ -17,4 +17,4 @@ pytest-datadir==1.3.1
 pytest-flake8==1.0.6
 pytest-mock==3.2.0
 pytest-pylint==0.17.0
-zest.releaser==6.20.1
+zest.releaser==6.21.0
