Add support for new Finnish identity codes

[Adman]

The documentation regarding the codes can be found at https://dvv.fi/en/reform-of-personal-identity-code

All Changes

• [x] Add an entry to the docs/changelog.rst describing the change.

• [x] Add an entry for your name in the docs/authors.rst file if it's not already there.

[claudep]

@Adman, could you please rebase? @benkonrath, do you think that apart from the Changelog update, this patch is OK?

[Adman]

@claudep Rebased

[benkonrath]

Looks good. Thanks!