`UseEnum` does not work with str enum

首先，使用 issue 中所示的 Color 枚举复现报告的 UseEnum 行为，然后检查 UseEnum 验证及其现有测试。为基于 str 的枚举添加覆盖，并验证分配底层值 "green" 会解析为 Color.GREEN，同时不破坏现有的枚举赋值。

Slightly adapting the UseEnum example to use a str based enum makes it fail to recognize the enum by the value:

import enum
from traitlets import HasTraits, UseEnum

class Color(str, enum.Enum):
    RED = "red"
    BLUE = "blue"
    GREEN = "green"

class MyEntity(HasTraits):
    color = UseEnum(Color, default_value=Color.BLUE)

entity = MyEntity(color=Color.RED)
entity.color = Color.GREEN
entity.color = "GREEN"  
entity.color = "Color.GREEN"  
entity.color = "green"
assert entity.color is Color.GREEN

Raises:

❯ python test_enum.py
Traceback (most recent call last):
  File "/home/mnoethe/Projects/test_enum.py", line 16, in <module>
    entity.color = "green"              # USE: number (as int)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 729, in __set__
    self.set(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 703, in set
    new_value = self._validate(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 735, in _validate
    value = self.validate(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 3582, in validate
    self.error(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 841, in error
    raise TraitError(e)
traitlets.traitlets.TraitError: The 'color' trait of a MyEntity instance expected any of ['RED', 'BLUE', 'GREEN'], not the str 'green'.