getattr syntax should support enum value as argument

This should be legal:

>>> from flufl.enum import make
>>> Colors = make('Colors', 'red green blue'.split())
>>> Colors[Colors.red]
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/flufl/enum/_enum.py", line 92, in __getitem__
    return getattr(cls, i)
TypeError: getattr(): attribute name must be string

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/flufl/enum/_enum.py", line 94, in __getitem__
    raise ValueError(i)
ValueError: Colors.red