Migrate to a package, add type information

Fix #287: Use DER by default

Update versions of dev packages

Bump version: 2.7.1 -> 2.8.0

Upgrade GH action version to remove warnings

Update change log

Bump version: 2.5.0 -> 2.6.0

Update changelog

Bump version: 2.4.1 -> 2.4.2

Bump version: 2.4.0 -> 2.4.1