Refactor get walletclass (#1776)

* move `get_wallet_class` to wallets module

* adjust imports, fix type issues

flake8

tests/core/views/test_api.py

@@ -9,7 +9,7 @@ from lnbits.core.models import Payment
 from lnbits.core.views.admin_api import api_auditor
 from lnbits.core.views.api import api_payment
 from lnbits.db import DB_TYPE, SQLITE
-from lnbits.settings import get_wallet_class
+from lnbits.wallets import get_wallet_class
 from tests.conftest import CreateInvoiceData, api_payments_create_invoice
 
 from ...helpers import get_random_invoice_data, is_fake, pay_real_invoice