Make vouchertype optional

There are validation rules in SETRA that sets this value to a default
value set in the parameters table of SETRA. For this to be possible
we must allow users to leave out the value.

setra_client/models.py

@@ -65,6 +65,6 @@ class Batch(BaseModel):
     batchid: str
     period: str
     interface: str
-    vouchertype: str
+    vouchertype: typing.Optional[str]
     vouchers: typing.List[Voucher]