Update isort requirement from <4.3.22,>=4.2.15 to >=4.2.15,<5.4.3

Updates the requirements on [isort](https://github.com/timothycrosley/isort) to permit the latest version.
- [Release notes](https://github.com/timothycrosley/isort/releases)
- [Changelog](https://github.com/timothycrosley/isort/blob/develop/CHANGELOG.md)
- [Commits](PyCQA/isort@4.2.15...5.4.2)

Signed-off-by: dependabot[bot] <[email protected]>

setup.py

@@ -21,7 +21,7 @@ def find_version(*file_paths):
 extras_require = {
     'linter': [
         "flake8==3.7.9",
-        "isort>=4.2.15,<4.3.22",
+        "isort>=4.2.15,<5.4.3",
     ],
     'dev': [
         "boto3==1.13.19",