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/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