bpo-38659: [Enum] add _simple_enum decorator (GH-25285)

add:

_simple_enum decorator to transform a normal class into an enum
_test_simple_enum function to compare
_old_convert_ to enable checking _convert_ generated enums
_simple_enum takes a normal class and converts it into an enum:

@simple_enum(Enum)
class Color:
    RED = 1
    GREEN = 2
    BLUE = 3

_old_convert_ works much like _convert_ does, using the original logic:

# in a test file
import socket, enum
CheckedAddressFamily = enum._old_convert_(
        enum.IntEnum, 'AddressFamily', 'socket',
        lambda C: C.isupper() and C.startswith('AF_'),
        source=_socket,
        )

test_simple_enum takes a traditional enum and a simple enum and
compares the two:

# in the REPL or the same module as Color
class CheckedColor(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

_test_simple_enum(CheckedColor, Color)

_test_simple_enum(CheckedAddressFamily, socket.AddressFamily)

Any important differences will raise a TypeError

Lib/test/test_socket.py

@@ -1941,6 +1941,41 @@ def test_socket_fileno_requires_socket_fd(self):
                     fileno=afile.fileno())
             self.assertEqual(cm.exception.errno, errno.ENOTSOCK)
 
+    def test_addressfamily_enum(self):
+        import _socket, enum
+        CheckedAddressFamily = enum._old_convert_(
+                enum.IntEnum, 'AddressFamily', 'socket',
+                lambda C: C.isupper() and C.startswith('AF_'),
+                source=_socket,
+                )
+        enum._test_simple_enum(CheckedAddressFamily, socket.AddressFamily)
+
+    def test_socketkind_enum(self):
+        import _socket, enum
+        CheckedSocketKind = enum._old_convert_(
+                enum.IntEnum, 'SocketKind', 'socket',
+                lambda C: C.isupper() and C.startswith('SOCK_'),
+                source=_socket,
+                )
+        enum._test_simple_enum(CheckedSocketKind, socket.SocketKind)
+
+    def test_msgflag_enum(self):
+        import _socket, enum
+        CheckedMsgFlag = enum._old_convert_(
+                enum.IntFlag, 'MsgFlag', 'socket',
+                lambda C: C.isupper() and C.startswith('MSG_'),
+                source=_socket,
+                )
+        enum._test_simple_enum(CheckedMsgFlag, socket.MsgFlag)
+
+    def test_addressinfo_enum(self):
+        import _socket, enum
+        CheckedAddressInfo = enum._old_convert_(
+                enum.IntFlag, 'AddressInfo', 'socket',
+                lambda C: C.isupper() and C.startswith('AI_'),
+                source=_socket)
+        enum._test_simple_enum(CheckedAddressInfo, socket.AddressInfo)
+
 
 @unittest.skipUnless(HAVE_SOCKET_CAN, 'SocketCan required for this test.')
 class BasicCANTest(unittest.TestCase):