gh-103056: [Enum] use staticmethod decorator for _gnv_ (GH-103231)

_gnv_ --> _generate_next_value_

Author: ethanfurman

Doc/howto/enum.rst

@@ -284,6 +284,7 @@ The values are chosen by :func:`_generate_next_value_`, which can be
 overridden::
 
     >>> class AutoName(Enum):
+    ...     @staticmethod
     ...     def _generate_next_value_(name, start, count, last_values):
     ...         return name
     ...

Lib/enum.py

@@ -1146,6 +1146,7 @@ def __new__(cls, value):
     def __init__(self, *args, **kwds):
         pass
 
+    @staticmethod
     def _generate_next_value_(name, start, count, last_values):
         """
         Generate the next value when not given.
@@ -1288,6 +1289,7 @@ def __new__(cls, *values):
         member._value_ = value
         return member
 
+    @staticmethod
     def _generate_next_value_(name, start, count, last_values):
         """
         Return the lower-cased version of the member name.
@@ -1337,6 +1339,7 @@ def __reduce_ex__(self, proto):
 
     _numeric_repr_ = repr
 
+    @staticmethod
     def _generate_next_value_(name, start, count, last_values):
         """
         Generate the next value when not given.