Fix dateformats

Setra expects datetime.datetime, not datetime.date In addition, voucherdate is in fact not optional, but transtype IS.

setra_client/models.py

@@ -46,7 +46,7 @@ class Transaction(BaseModel):
     dim7: Optional[str]
     sequenceno: Optional[int]
     taxcode: Optional[str]
-    transtype: str
+    transtype: Optional[str]
     extinvref: Optional[str]