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/tkinter/__init__.py

@@ -144,7 +144,8 @@ def _splitdict(tk, v, cut_minus=True, conv=None):
     return dict
 
 
-class EventType(enum.StrEnum):
+@enum._simple_enum(enum.StrEnum)
+class EventType:
     KeyPress = '2'
     Key = KeyPress
     KeyRelease = '3'
@@ -185,8 +186,6 @@ class EventType(enum.StrEnum):
     Deactivate = '37'
     MouseWheel = '38'
 
-    __str__ = str.__str__
-
 
 class Event:
     """Container for the properties of an event.