Bump isort from 4.3.20 to 4.3.21 in /{{cookiecutter.app_name}}

Bumps [isort](https://github.com/timothycrosley/isort) from 4.3.20 to 4.3.21.
- [Release notes](https://github.com/timothycrosley/isort/releases)
- [Changelog](https://github.com/timothycrosley/isort/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/timothycrosley/isort/compare/4.3.20...4.3.21)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

{{cookiecutter.app_name}}/Pipfile

@@ -55,5 +55,5 @@ flake8-blind-except = "==0.1.1"
 flake8-debugger = "==3.1.0"
 flake8-docstrings = "==1.3.0"
 flake8-isort = "==2.7.0"
-isort = "==4.3.20"
+isort = "==4.3.21"
 pep8-naming = "==0.8.2"