Require Python 3.6 and use isort >= 5

isort 5.x requires Python >= 3.6, so let's update the requirement
as well.

.travis.yml

@@ -18,7 +18,6 @@ language: python
 cache: pip
 
 python:
-  - "3.5"
   - "3.6"
   - "3.7"
   - "3.8"

README.dev.md

@@ -12,7 +12,7 @@ and uses Mezzanine (http://mezzanine.jupo.org/) for content management.
 As Django is based on Python, first of all you need Python and a couple of
 extra packages installed on your system.
 
-The code requires Python 3.5 or higher.
+The code requires Python 3.6 or higher.
 
 All Python related dependencies are listed in `requirements.txt` and can
 be installed via `pip` (see below).

tox.ini

@@ -22,13 +22,13 @@ geany_modules = geany latest_version news nightlybuilds pastebin static_docs
 [testenv]
 deps =
     flake8
-    isort<5  # temporarily stick to isort 4.x: https://github.com/PyCQA/pylint/pull/3725
+    isort
     pylint
     pylint-django
     -r{toxinidir}/requirements.txt
 commands =
     {envbindir}/flake8 {[tox]geany_modules}
-    {envbindir}/isort --check-only --diff --recursive {[tox]geany_modules}
+    {envbindir}/isort --check-only --diff {[tox]geany_modules}
     {envbindir}/pylint --rcfile=tox.ini {[tox]geany_modules}
 
 [flake8]