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/nostr/filter.py

@@ -76,7 +76,7 @@ class Filter:
             return False
 
         if self.tags:
-            e_tag_identifiers = set([e_tag[0] for e_tag in event.tags])
+            e_tag_identifiers = {e_tag[0] for e_tag in event.tags}
             for f_tag, f_tag_values in self.tags.items():
                 # Omit any NIP-01 or NIP-12 "#" chars on single-letter tags
                 f_tag = f_tag.replace("#", "")