Updates to support isort 5.0

Removed `not_skip` and `recursive` from `tox.ini` to fix
TypeError: __init__() got an unexpected keyword argument 'not_skip'

* recursive
Prior to version 5.0.0, isort wouldn't automatically traverse directories. The --recursive option was necessary to tell it to do so. In 5.0.0 directories are automatically traversed for all Python files, and as such this option is no longer necessary and should simply be removed.

* not_skip
In an earlier version isort had a default skip of __init__.py. To get around that many projects wanted a way to not skip __init__.py or any other files that were automatically skipped in the future by isort. isort no longer has any default skips, so if the value here is __init__.py you can simply remove the setting.

https://timothycrosley.github.io/isort/docs/upgrade_guides/5.0.0/

tox.ini

@@ -33,7 +33,7 @@ deps=
 commands=
     black --check treemodeladmin setup.py
     flake8 .
-    isort --check-only --diff --recursive treemodeladmin
+    isort --check-only --diff treemodeladmin
 
 [flake8]
 ignore=E731,W503,W504
@@ -49,7 +49,6 @@ lines_after_imports=2
 include_trailing_comma=1
 multi_line_output=3
 skip=.tox,migrations
-not_skip=__init__.py
 use_parentheses=1
 known_django=django
 known_wagtail=wagtail