Made two batch fields optional, because sometimes when you retrieve a batch...

it will not fill these field, because they are not yet available, and therefore it would fail when parsing the data 

setra_client/models.py

@@ -194,10 +194,10 @@ class CompleteBatch(BaseModel):
     """Model for the /batch_complete/<id> endpoint"""
     created: datetime.datetime
     batchid: str
-    period: int
+    period: Optional[int]
     interface: str
     client: str
-    vouchertype: str
+    vouchertype: Optional[str]
     batch_validated_ok_date: Optional[datetime.datetime]
     batch_rejected_code: Optional[str]
     sent_date: Optional[datetime.datetime]