### Issue:
In DATEV customer and supplier exports, partners outside the European Union still had the `EU-Land` and `EU-UStID` fields filled

However, these fields must only be used for EU countries

For non-EU countries, the `Land` field should be filled instead, and is required whenever the country is not Germany https://developer.datev.de/en/file-format/details/datev-format/format-description/debitorskreditors

### Cause:
`_l10n_de_datev_get_partner_list` did not distinguish between EU and non-EU countries

As a result, any partner with a VAT number could populate `EU-Land` and `EU-UStID`, even if the country was outside the EU

Greece also requires a special case: its VAT prefix is `EL` so the `EU-Land` too, while the country code used in `Land` must remain `GR`

### Steps to reproduce:
- Install `l10n_de_reports` and switch to the DE company
- Create a customer in Switzerland with a valid VAT number
- Create and confirm an invoice for that customer
- Go to Accounting → Audit Reports → General Ledger
- Select the full year
- From the gear menu, export DATEV DATA (zip)
- Open the `EXTF_customer_accounts` file

### Before the fix:
`EU-Land` and `EU-UStID` are filled for the Swiss customer, while `Land` is empty

### After the fix:
`EU-Land` and `EU-UStID` are empty for non-EU countries such as Switzerland, while `Land` is correctly filled

`Land` is filled using the following priority:
1. Partner country_code
2. Country extracted from the VAT number
3. Empty

opw-5902565