chore: run pyupgrade (#623)

- use `{...}` instead of `set([...])`
- do not use `class Foo(object):`, just use `class Foo:`
- do not specify default flags (`"r"`) for `open()`

cashu/mint/verification.py

@@ -234,7 +234,7 @@ class LedgerVerification(
         return units_proofs[0]
 
     def get_fees_for_proofs(self, proofs: List[Proof]) -> int:
-        if not len(set([self.keysets[p.id].unit for p in proofs])) == 1:
+        if not len({self.keysets[p.id].unit for p in proofs}) == 1:
             raise TransactionUnitError("inputs have different units.")
         fee = (sum([self.keysets[p.id].input_fee_ppk for p in proofs]) + 999) // 1000
         return fee