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/tkinter/test/test_tkinter/test_misc.py

@@ -1,5 +1,6 @@
 import unittest
 import tkinter
+import enum
 from test import support
 from tkinter.test.support import AbstractTkTest, AbstractDefaultRootTest
 
@@ -261,6 +262,49 @@ class MiscTest(AbstractTkTest, unittest.TestCase):
                          " num=3 delta=-1 focus=True"
                          " x=10 y=20 width=300 height=200>")
 
+    def test_eventtype_enum(self):
+        class CheckedEventType(enum.StrEnum):
+            KeyPress = '2'
+            Key = KeyPress
+            KeyRelease = '3'
+            ButtonPress = '4'
+            Button = ButtonPress
+            ButtonRelease = '5'
+            Motion = '6'
+            Enter = '7'
+            Leave = '8'
+            FocusIn = '9'
+            FocusOut = '10'
+            Keymap = '11'           # undocumented
+            Expose = '12'
+            GraphicsExpose = '13'   # undocumented
+            NoExpose = '14'         # undocumented
+            Visibility = '15'
+            Create = '16'
+            Destroy = '17'
+            Unmap = '18'
+            Map = '19'
+            MapRequest = '20'
+            Reparent = '21'
+            Configure = '22'
+            ConfigureRequest = '23'
+            Gravity = '24'
+            ResizeRequest = '25'
+            Circulate = '26'
+            CirculateRequest = '27'
+            Property = '28'
+            SelectionClear = '29'   # undocumented
+            SelectionRequest = '30' # undocumented
+            Selection = '31'        # undocumented
+            Colormap = '32'
+            ClientMessage = '33'    # undocumented
+            Mapping = '34'          # undocumented
+            VirtualEvent = '35'     # undocumented
+            Activate = '36'
+            Deactivate = '37'
+            MouseWheel = '38'
+        enum._test_simple_enum(CheckedEventType, tkinter.EventType)
+
     def test_getboolean(self):
         for v in 'true', 'yes', 'on', '1', 't', 'y', 1, True:
             self.assertIs(self.root.getboolean(v), True)