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/lightning/lnd_grpc/lnd_grpc.py

@@ -49,7 +49,7 @@ INVOICE_STATUSES = {
 class LndRPCWallet(LightningBackend):
     supports_mpp = settings.mint_lnd_enable_mpp
     supports_incoming_payment_stream = True
-    supported_units = set([Unit.sat, Unit.msat])
+    supported_units = {Unit.sat, Unit.msat}
     supports_description: bool = True
 
     unit = Unit.sat