remove support for Django 2.1

Author: PetrDlouhy

Diff for: CHANGELOG.md

@@ -10,6 +10,8 @@ If your database contains colliding slugs, they will be automatically renamed by
 Three categories with slugs ``foo`` will be renamed to ``foo``, ``foo-1``, ``foo-2``.
 If this causes problems in your project, you can rename the categories yourself before running the migration.
 
+- Django 2.1 is no longer supported
+
 
 ## 1.9.2 (2022-09-22)
 [Compare the full difference.](https://github.com/jazzband/django-categories/compare/1.9.1...1.9.2)

Diff for: tox.ini

@@ -2,7 +2,6 @@
 envlist =
     begin
     py37-lint
-    py{37}-django{21}
     py{37,38,39}-django{22,3,31}
     py{37,38,39,310}-django{32}
     py{38,39,310}-django{40}
@@ -22,8 +21,6 @@ python =
 passenv = GITHUB_*
 
 deps=
-    django2: Django>=2.0,<2.1
-    django21: Django>=2.1,<2.2
     django22: Django>=2.2,<2.3
     django3: Django>=3.0,<3.1
     django31: Django>=3.1,<3.2