fix: StringEnum._generate_next_value_ signature (#3770)

per the documentation, `_generate_next_value_` should be a staticmethod.

reference:
https://docs.python.org/3/library/enum.html#enum.Enum._generate_next_value_

vyper/utils.py

@@ -65,6 +65,7 @@ def copy(self):
 
 class StringEnum(enum.Enum):
     # Must be first, or else won't work, specifies what .value is
+    @staticmethod
     def _generate_next_value_(name, start, count, last_values):
         return name.lower()