Drop support for Python 2 in template generation hooks

[browniebroke]

Description

Python 2.7 has been EOL for 5 years, it's time. This PR is not fixing all the issues but remove the comment at the top of each file as well as the most obvious changes. The rest will be fixed either automatically in #5613 or manually as a follow up change.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

Enable us to use modern syntax in prr/post-generation hooks