Improve enums (#3168)

I realized while working on srittau/type-stub-pep#64 that a
few things we do in existing enum definitions in typeshed are
problematic:

- Using "= ..." doesn't allow type checkers to correctly type the
  result of Enum.MEMBER.value. In fact, mypy at least infers
  .value to be "Ellipsis" if you do this.
- Properties on the enum values themselves, like HTTPStatus.phrase,
  should not be specified directly as attributes, because it makes
  type checkers think that the properties themselves are enum
  members.

I ended up doing a bit more cleanup to the signal module:
- Remove unnecessary ... initializers.
- Remove unnecessary _SIG = Signals alias.
- I don't have Windows to test, but the C code for _signal suggests
  that CTRL_C_EVENT and CTRL_BREAK events are not Signals, but just ints:
  1dbd084f1f/Modules/signalmodule.c (L1575)

stdlib/3/http/__init__.pyi

@@ -1,67 +1,65 @@
-import sys
-
 from enum import IntEnum
 
 class HTTPStatus(IntEnum):
-    def __init__(self, *a) -> None: ...
+    @property
+    def phrase(self) -> str: ...
+    @property
+    def description(self) -> str: ...
 
-    phrase: str
-    description: str
-
-    CONTINUE = ...
-    SWITCHING_PROTOCOLS = ...
-    PROCESSING = ...
-    OK = ...
-    CREATED = ...
-    ACCEPTED = ...
-    NON_AUTHORITATIVE_INFORMATION = ...
-    NO_CONTENT = ...
-    RESET_CONTENT = ...
-    PARTIAL_CONTENT = ...
-    MULTI_STATUS = ...
-    ALREADY_REPORTED = ...
-    IM_USED = ...
-    MULTIPLE_CHOICES = ...
-    MOVED_PERMANENTLY = ...
-    FOUND = ...
-    SEE_OTHER = ...
-    NOT_MODIFIED = ...
-    USE_PROXY = ...
-    TEMPORARY_REDIRECT = ...
-    PERMANENT_REDIRECT = ...
-    BAD_REQUEST = ...
-    UNAUTHORIZED = ...
-    PAYMENT_REQUIRED = ...
-    FORBIDDEN = ...
-    NOT_FOUND = ...
-    METHOD_NOT_ALLOWED = ...
-    NOT_ACCEPTABLE = ...
-    PROXY_AUTHENTICATION_REQUIRED = ...
-    REQUEST_TIMEOUT = ...
-    CONFLICT = ...
-    GONE = ...
-    LENGTH_REQUIRED = ...
-    PRECONDITION_FAILED = ...
-    REQUEST_ENTITY_TOO_LARGE = ...
-    REQUEST_URI_TOO_LONG = ...
-    UNSUPPORTED_MEDIA_TYPE = ...
-    REQUESTED_RANGE_NOT_SATISFIABLE = ...
-    EXPECTATION_FAILED = ...
-    UNPROCESSABLE_ENTITY = ...
-    LOCKED = ...
-    FAILED_DEPENDENCY = ...
-    UPGRADE_REQUIRED = ...
-    PRECONDITION_REQUIRED = ...
-    TOO_MANY_REQUESTS = ...
-    REQUEST_HEADER_FIELDS_TOO_LARGE = ...
-    INTERNAL_SERVER_ERROR = ...
-    NOT_IMPLEMENTED = ...
-    BAD_GATEWAY = ...
-    SERVICE_UNAVAILABLE = ...
-    GATEWAY_TIMEOUT = ...
-    HTTP_VERSION_NOT_SUPPORTED = ...
-    VARIANT_ALSO_NEGOTIATES = ...
-    INSUFFICIENT_STORAGE = ...
-    LOOP_DETECTED = ...
-    NOT_EXTENDED = ...
-    NETWORK_AUTHENTICATION_REQUIRED = ...
+    CONTINUE: int
+    SWITCHING_PROTOCOLS: int
+    PROCESSING: int
+    OK: int
+    CREATED: int
+    ACCEPTED: int
+    NON_AUTHORITATIVE_INFORMATION: int
+    NO_CONTENT: int
+    RESET_CONTENT: int
+    PARTIAL_CONTENT: int
+    MULTI_STATUS: int
+    ALREADY_REPORTED: int
+    IM_USED: int
+    MULTIPLE_CHOICES: int
+    MOVED_PERMANENTLY: int
+    FOUND: int
+    SEE_OTHER: int
+    NOT_MODIFIED: int
+    USE_PROXY: int
+    TEMPORARY_REDIRECT: int
+    PERMANENT_REDIRECT: int
+    BAD_REQUEST: int
+    UNAUTHORIZED: int
+    PAYMENT_REQUIRED: int
+    FORBIDDEN: int
+    NOT_FOUND: int
+    METHOD_NOT_ALLOWED: int
+    NOT_ACCEPTABLE: int
+    PROXY_AUTHENTICATION_REQUIRED: int
+    REQUEST_TIMEOUT: int
+    CONFLICT: int
+    GONE: int
+    LENGTH_REQUIRED: int
+    PRECONDITION_FAILED: int
+    REQUEST_ENTITY_TOO_LARGE: int
+    REQUEST_URI_TOO_LONG: int
+    UNSUPPORTED_MEDIA_TYPE: int
+    REQUESTED_RANGE_NOT_SATISFIABLE: int
+    EXPECTATION_FAILED: int
+    UNPROCESSABLE_ENTITY: int
+    LOCKED: int
+    FAILED_DEPENDENCY: int
+    UPGRADE_REQUIRED: int
+    PRECONDITION_REQUIRED: int
+    TOO_MANY_REQUESTS: int
+    REQUEST_HEADER_FIELDS_TOO_LARGE: int
+    INTERNAL_SERVER_ERROR: int
+    NOT_IMPLEMENTED: int
+    BAD_GATEWAY: int
+    SERVICE_UNAVAILABLE: int
+    GATEWAY_TIMEOUT: int
+    HTTP_VERSION_NOT_SUPPORTED: int
+    VARIANT_ALSO_NEGOTIATES: int
+    INSUFFICIENT_STORAGE: int
+    LOOP_DETECTED: int
+    NOT_EXTENDED: int
+    NETWORK_AUTHENTICATION_REQUIRED: int