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_signal.py

@@ -1,3 +1,4 @@
+import enum
 import errno
 import os
 import random
@@ -33,6 +34,32 @@ class GenericTests(unittest.TestCase):
                 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):