Use isort 4.x

Pylint is not yet compatible with isort 5.x
For more information please visit:
  * pylint-dev/pylint#3722
  * pylint-dev/pylint#3725

Signed-off-by: Jürgen Löhel <juergen.loehel@inlyse.com>

tox.ini

@@ -13,7 +13,7 @@ logstash_async_modules = logstash_async tests
 [testenv]
 deps =
     flake8
-    isort
+    isort<5
     pylint
 commands =
     # linting and code analysis
@@ -29,7 +29,7 @@ commands =
 # for Python 2.7.
 deps =
     flake8
-    isort
+    isort<5
 commands =
     # linting and code analysis
     {envbindir}/flake8 {[tox]logstash_async_modules}