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_ast.py

@@ -1,6 +1,7 @@
 import ast
 import builtins
 import dis
+import enum
 import os
 import sys
 import types
@@ -698,6 +699,35 @@ class AST_Tests(unittest.TestCase):
             with self.assertRaisesRegex(ValueError, f"Name node can't be used with '{constant}' constant"):
                 compile(expr, "<test>", "eval")
 
+    def test_precedence_enum(self):
+        class _Precedence(enum.IntEnum):
+            """Precedence table that originated from python grammar."""
+            TUPLE = enum.auto()
+            YIELD = enum.auto()           # 'yield', 'yield from'
+            TEST = enum.auto()            # 'if'-'else', 'lambda'
+            OR = enum.auto()              # 'or'
+            AND = enum.auto()             # 'and'
+            NOT = enum.auto()             # 'not'
+            CMP = enum.auto()             # '<', '>', '==', '>=', '<=', '!=',
+                                          # 'in', 'not in', 'is', 'is not'
+            EXPR = enum.auto()
+            BOR = EXPR                    # '|'
+            BXOR = enum.auto()            # '^'
+            BAND = enum.auto()            # '&'
+            SHIFT = enum.auto()           # '<<', '>>'
+            ARITH = enum.auto()           # '+', '-'
+            TERM = enum.auto()            # '*', '@', '/', '%', '//'
+            FACTOR = enum.auto()          # unary '+', '-', '~'
+            POWER = enum.auto()           # '**'
+            AWAIT = enum.auto()           # 'await'
+            ATOM = enum.auto()
+            def next(self):
+                try:
+                    return self.__class__(self + 1)
+                except ValueError:
+                    return self
+        enum._test_simple_enum(_Precedence, ast._Precedence)
+
 
 class ASTHelpers_Test(unittest.TestCase):
     maxDiff = None