Structure and native Structure (LittleEndianStructure on Windows) supports __slots__, but BigEndianStructure doesn't

[0b4809ce-f77d-40b4-a79a-6012870dabba]

BPO	16070
Files	slots_test.py: test script

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['ctypes', 'type-bug'] title = "Structure and native Structure (LittleEndianStructure on Windows) supports __slots__, but BigEndianStructure doesn't" updated_at = user = 'https://bugs.python.org/hct' ``` bugs.python.org fields: ```python activity = actor = 'BreamoreBoy' assignee = 'none' closed = False closed_date = None closer = None components = ['ctypes'] creation = creator = 'hct' dependencies = [] files = ['27323'] hgrepos = [] issue_num = 16070 keywords = [] message_count = 2.0 messages = ['171402', '222928'] nosy_count = 1.0 nosy_names = ['hct'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue16070' versions = ['Python 3.4', 'Python 3.5'] ```

[0b4809ce-f77d-40b4-a79a-6012870dabba]

using official CPython 3.2.3 with a simple demonstration script (attached) to demonstrate inconsistency between ctypes structures

from ctypes import *

class cbs( BigEndianStructure ):
    __slots__ = tuple()
    def __init__( self, *args, **kw ):
        super().__init__( *args, **kw )
        self.a = 11

class cls( LittleEndianStructure ):
    __slots__ = tuple()
    def __init__( self, *args, **kw ):
        super().__init__( *args, **kw )
        self.a = 11

class cs( Structure ):
    __slots__ = tuple()
    def __init__( self, *args, **kw ):
        super().__init__( *args, **kw )
        self.a = 11

try : cbs1=cbs() except AttributeError as e: print(e)

try : cls1=cls() except AttributeError as e: print(e)

try : cs=cs() except AttributeError as e: print(e)

yields

'cls' object has no attribute 'a' 'cs' object has no attribute 'a'

I expect cbs to throw error too, but itwent through the initalization and silently ignored the __slots__ defined in the class

[83d2e70e-e599-4a04-b820-3814bbdb9bef]

@hct I'm very sorry about the delay in getting back to you. The reported bevaviour is the same with 3.4.1 and 3.5.0a0 on Windows 7.