ci: update isort

Before removing Py2 we were already using isort 3.8.0 in the tests, as
we were requiring isort 5 explicitly:
```
pip install 'isort==5.*' black
```
ce83709 starts using the setup.py
version, which makes the tests fail.

Note this was not a problem because we were using Py3 for the code
linters.

setup.py

@@ -83,7 +83,7 @@
             "pytest-cov==2.11.1",
             "pycodestyle==2.7.0",
             "black==20.8b1 ; python_version>'3.0'",
-            "isort==4.3.21",  # TODO: Change to 5.8.0 when removing py2
+            "isort==5.8.0 ; python_version>'3.0'",
         ]
     },
     zip_safe=False,