Check the amount of base types in `Enum`

[sobolevn]

Right now this code does not produce any issues in mypy:

import enum

class First:
    def __new__(cls, val):
        return 1

class Second:
    def __new__(cls, val):
        return 2

class Some(First, Second, enum.Enum):  # ok
    x = 1

But, in runtime it fails:

>>> import enum
>>> 
>>> class First:
...     def __new__(cls, val):
...         return 1
... 
>>> class Second:
...     def __new__(cls, val):
...         return 2
... 
>>> class Some(First, Second, enum.Enum):
...     x = 1
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sobolev/Desktop/cpython/Lib/enum.py", line 403, in __prepare__
    member_type, first_enum = metacls._get_mixins_(cls, bases)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/Lib/enum.py", line 893, in _get_mixins_
    member_type = _find_data_type(bases) or object
                  ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/Lib/enum.py", line 881, in _find_data_type
    raise TypeError('%r: too many data types: %r' % (class_name, data_types))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: 'Some': too many data types: {<class '__main__.Second'>, <class '__main__.First'>}

This happens because Enum can have only one extra subclass with __new__, because it is used to create new Enum members.