Beancount-ing

- Add Beancount 3.x support (thus removing Beancount 2.x support)
- Add `beancount-ing-ec` CLI command
- Rename `account` parameter to `account_name` (overlapping with the `account()` method
definition required by `beangulp.importer.Importer`)
- Add Python 3.11 and 3.12 support
- Drop Python 3.7 support

- Rename to `beancount-ing`

- Generate `Balance` directives for opening / closing dates (thanks [szabootibor])

- Handle duplicate "Währung" field name (thanks [codedump] for the heads-up)
- Allow optional ";Letztes Update: aktuell" line in the header (thanks [szabootibor])

- Support "Kategorie" field in the CSV downloads
- Enable support for Python 3.9
- Drop support for Python 3.5

- Update `extract` interface