Make Builder a subclass of abc.ABC

[AA-Turner]

Closes #12996

cc @chrisjsewell, having written the PR I'm not sure it's worth it. What do you think?

A

[chrisjsewell]

I think it makes sense; harder to mess up writing a builder 🤷 (also I made #12998)

[AA-Turner]

The slight challenge is that if you use write_documents() then write_doc and get_target_uri aren't required if you don't use them, which could be annoying.