Use local import to prevent import errors. (#78)

Author: Kevin Hellemun

bunq/sdk/context.py

@@ -7,7 +7,6 @@
 from bunq.sdk.exception import BunqException
 from bunq.sdk.json import converter
 from bunq.sdk.model import core
-from bunq.sdk.model.device_server_internal import DeviceServerInternal
 from bunq.sdk.model.generated import endpoint
 
 
@@ -121,6 +120,8 @@ def _register_device(self, device_description,
         :rtype: None
         """
 
+        from bunq.sdk.model.device_server_internal import DeviceServerInternal
+
         DeviceServerInternal.create(
             device_description,
             self.api_key,