Specify a minimum attrs version of 21.4.0

Author: meshy

CHANGELOG.md

@@ -11,6 +11,7 @@ Version numbers should follow https://semver.org/spec/v2.0.0.html
 
 - Python 3.10 tests now only run against Django 3.2+, because Django <3.2 does
   not support Python 3.10.
+- Specify a minimum `attrs` version of `21.4.0`.
 
 ## [2.1.0] - 2022-08-01
 

pyproject.toml

@@ -39,7 +39,7 @@ classifiers = [
 
 [tool.poetry.dependencies]
 python = ">=3.6,<3.11"
-attrs = ""
+attrs = "^21.4.0"
 
 [tool.poetry.dev-dependencies]
 django-environ = "^0.4.5"