bpo-38659: [Enum] add _simple_enum decorator (GH-25497)

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 @@ class GeneralModuleTests(unittest.TestCase):
                     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):