Make Batch.period optional

Validation rule 12 on Batches in SETRA sets the default value before
transfer to Unit4ERP(UBW) if it is not set. The attribute is thus not
required.

setra_client/models.py

@@ -60,11 +60,13 @@ class Voucher(BaseModel):
 
 
 class Batch(BaseModel):
-    """Model representing a batch, with a list of vouchers (and each voucher has a list of transactions)"""
+    """
+    Model representing a batch, with a list of vouchers (and each
+    voucher has a list of transactions)
+    """
     client: str
     batchid: str
-    period: str
+    period: typing.Optional[str]
     interface: str
     vouchertype: typing.Optional[str]
     vouchers: typing.List[Voucher]
-