Use enum trick for dataclasses.MISSING (#7127)

The goal of this change is to fix the behavior of
`dataclasses.Field`. Several attributes of a `Field` may have a value
of `MISSING` (a sentinel value). As a result, attributes of Field may
be checked against `MISSING` as in

     [f for f in fields(obj) if f.default is MISSING]

`default`, `default_factory`, and `kw_only` are the attributes
which may have a value of `MISSING`.

This workaround of defining `_MISSING_TYPE` as an enum, and `MISSING`
as its only member, allows `... | Literal[_MISSING_TYPE.MISSING]` to
be used in type annotations for these attributes. This is very
slightly inaccurate -- primarily in that `_MISSING_TYPE` isn't really
an enum -- but this allows for use in `Literal`.
After PEP 661 (Sentinel Values), there may be a more accurate way of
writing this, but for now this works.

This adjustment is only applied to the attributes of Field, not the
arguments to functions and methods.

stdlib/dataclasses.pyi

@@ -1,7 +1,9 @@
+import enum
 import sys
 import types
 from builtins import type as Type  # alias to avoid name clashes with fields named "type"
 from typing import Any, Callable, Generic, Iterable, Mapping, Protocol, TypeVar, overload
+from typing_extensions import Literal
 
 if sys.version_info >= (3, 9):
     from types import GenericAlias
@@ -9,9 +11,15 @@ if sys.version_info >= (3, 9):
 _T = TypeVar("_T")
 _T_co = TypeVar("_T_co", covariant=True)
 
-class _MISSING_TYPE: ...
+# define _MISSING_TYPE as an enum within the type stubs,
+# even though that is not really its type at runtime
+# this allows us to use Literal[_MISSING_TYPE.MISSING]
+# for background, see:
+#   https://github.com/python/typeshed/pull/5900#issuecomment-895513797
+class _MISSING_TYPE(enum.Enum):
+    MISSING = enum.auto()
 
-MISSING: _MISSING_TYPE
+MISSING = _MISSING_TYPE.MISSING
 
 if sys.version_info >= (3, 10):
     class KW_ONLY: ...
@@ -72,15 +80,15 @@ class _DefaultFactory(Protocol[_T_co]):
 class Field(Generic[_T]):
     name: str
     type: Type[_T]
-    default: _T
-    default_factory: _DefaultFactory[_T]
+    default: _T | Literal[_MISSING_TYPE.MISSING]
+    default_factory: _DefaultFactory[_T] | Literal[_MISSING_TYPE.MISSING]
     repr: bool
     hash: bool | None
     init: bool
     compare: bool
     metadata: types.MappingProxyType[Any, Any]
     if sys.version_info >= (3, 10):
-        kw_only: bool
+        kw_only: bool | Literal[_MISSING_TYPE.MISSING]
         def __init__(
             self,
             default: _T,