Invoicing: Allow to show exchange rates based on sources/rules (#3122)

src/pretix/base/models/__init__.py

@@ -23,6 +23,7 @@ from ..settings import GlobalSettingsObject_SettingsStore
 from .auth import U2FDevice, User, WebAuthnDevice
 from .base import CachedFile, LoggedModel, cachedfile_name
 from .checkin import Checkin, CheckinList
+from .currencies import ExchangeRate
 from .customers import Customer
 from .devices import Device, Gate
 from .discount import Discount