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_unicode.py

@@ -1463,20 +1463,21 @@ class UnicodeTest(string_tests.CommonTest,
             PI = 3.1415926
         class Int(enum.IntEnum):
             IDES = 15
-        class Str(str, enum.Enum):
+        class Str(enum.StrEnum):
+            # StrEnum uses the value and not the name for %s etc.
             ABC = 'abc'
         # Testing Unicode formatting strings...
         self.assertEqual("%s, %s" % (Str.ABC, Str.ABC),
-                         'ABC, ABC')
+                         'abc, abc')
         self.assertEqual("%s, %s, %d, %i, %u, %f, %5.2f" %
                         (Str.ABC, Str.ABC,
                          Int.IDES, Int.IDES, Int.IDES,
                          Float.PI, Float.PI),
-                         'ABC, ABC, 15, 15, 15, 3.141593,  3.14')
+                         'abc, abc, 15, 15, 15, 3.141593,  3.14')
 
         # formatting jobs delegated from the string implementation:
         self.assertEqual('...%(foo)s...' % {'foo':Str.ABC},
-                         '...ABC...')
+                         '...abc...')
         self.assertEqual('...%(foo)s...' % {'foo':Int.IDES},
                          '...IDES...')
         self.assertEqual('...%(foo)i...' % {'foo':Int.IDES},