🔧 MAINTAIN: Update attrs minimum version

`value_serializer` has been supported since this version
(see: python-attrs/attrs#653)

Author: chrisjsewell

setup.cfg

@@ -29,7 +29,7 @@ project_urls =
 [options]
 packages = find:
 install_requires =
-    attrs>=19.2,<22
+    attrs>=20.3,<22
     click~=7.1
     pyyaml
     sphinx>=3,<5