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_signal.py

@@ -1,3 +1,4 @@
+import enum
 import errno
 import os
 import random
@@ -33,6 +34,32 @@ def test_enums(self):
                 self.assertIsInstance(sig, signal.Signals)
                 self.assertEqual(sys.platform, "win32")
 
+        CheckedSignals = enum._old_convert_(
+                enum.IntEnum, 'Signals', 'signal',
+                lambda name:
+                    name.isupper()
+                    and (name.startswith('SIG') and not name.startswith('SIG_'))
+                    or name.startswith('CTRL_'),
+                source=signal,
+                )
+        enum._test_simple_enum(CheckedSignals, signal.Signals)
+
+        CheckedHandlers = enum._old_convert_(
+                enum.IntEnum, 'Handlers', 'signal',
+                lambda name: name in ('SIG_DFL', 'SIG_IGN'),
+                source=signal,
+                )
+        enum._test_simple_enum(CheckedHandlers, signal.Handlers)
+
+        Sigmasks = getattr(signal, 'Sigmasks', None)
+        if Sigmasks is not None:
+            CheckedSigmasks = enum._old_convert_(
+                    enum.IntEnum, 'Sigmasks', 'signal',
+                    lambda name: name in ('SIG_BLOCK', 'SIG_UNBLOCK', 'SIG_SETMASK'),
+                    source=signal,
+                    )
+            enum._test_simple_enum(CheckedSigmasks, Sigmasks)
+
 
 @unittest.skipIf(sys.platform == "win32", "Not valid on Windows")
 class PosixTests(unittest.TestCase):