add type annotations in beangulp identify/extract/archive

[jokester]

Hi I learned how to write my own importers by reading the code (btw it was intuitive).

These changes are by-products but I thought they may help others too.

[blais]

Hmm, lots of failure, various reasons.

[jokester]

I added a commit to fix errors I could understand. Can someone approve the CI run again?

[jokester]

There was a lint error in code I didn't touch. Should this be fixed too?

https://github.com/beancount/beangulp/actions/runs/13861040615/job/38936900664

 beangulp/importers/config_test.py:24:31: C420 [*] Unnecessary dict comprehension for iterable; use `dict.fromkeys` instead
   |
23 |       def setUp(self):
24 |           self.default_config = {key: 'Assets:Something'
   |  _______________________________^
25 | |                                for key in SimpleTestImporter.REQUIRED_CONFIG}
   | |_____________________________________________________________________________^ C420
26 |
27 |       def test_constructors(self):
   |
   = help: Replace with `dict.fromkeys(iterable)`)

[jokester]

I think I fixed them all, please run again 🙏🏽

[blais]

Thank you!