Fix Enum field by-name lookup to only return actual members

[bysiber]

Summary

The Enum field's by-name deserialization uses getattr(self.enum, val) to look up members. However, getattr returns any attribute of the Enum class, not just actual enum members. This means inputs like "mro", "__class__", or "__members__" silently return non-Enum objects instead of raising a validation error.

Problem

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2

# Current behavior - these don't raise errors:
getattr(Color, "mro")         # <built-in method mro>
getattr(Color, "__members__")  # mappingproxy({...})
getattr(Color, "__class__")    # <class 'EnumType'>

When a user submits one of these attribute names as input, the Enum field returns the raw attribute/method object instead of an Enum member. Downstream code expecting an Enum member would break or behave unpredictably.

Fix

Replace getattr(self.enum, val) with self.enum[val]. The [] operator on Enum classes only looks up actual members (via EnumMeta.__getitem__), so non-member names correctly raise KeyError, which we catch and convert to a validation error.

# Before
getattr(Color, "mro")     # returns <built-in method mro>

# After  
Color["mro"]               # raises KeyError
Color["RED"]               # returns Color.RED ✓

[sloria]

Thanks! Mind adding a test and adding yourself to AUTHORS.rst?