Fold remaining custom stdlib *Buffer types into _typeshed. (#6082)

Add ctypes base type to WriteableBuffer.
Add a ReadOnlyBuffer type from fcntl.
Base ReadableBuffer on WriteableBuffer and ReadOnlyBuffer.
Use these types in fcntl and ctypes stubs.

stdlib/ctypes/__init__.pyi

@@ -1,5 +1,5 @@
 import sys
-from array import array
+from _typeshed import ReadableBuffer, WriteableBuffer
 from typing import (
     Any,
     Callable,
@@ -72,13 +72,6 @@ if sys.platform == "win32":
 pydll: LibraryLoader[PyDLL]
 pythonapi: PyDLL
 
-# Anything that implements the read-write buffer interface.
-# The buffer interface is defined purely on the C level, so we cannot define a normal Protocol
-# for it. Instead we have to list the most common stdlib buffer classes in a Union.
-_WritableBuffer = _UnionT[bytearray, memoryview, array[Any], _CData]
-# Same as _WritableBuffer, but also includes read-only buffer types (like bytes).
-_ReadOnlyBuffer = _UnionT[_WritableBuffer, bytes]
-
 class _CDataMeta(type):
     # By default mypy complains about the following two methods, because strictly speaking cls
     # might not be a Type[_CT]. However this can never actually happen, because the only class that
@@ -91,9 +84,9 @@ class _CData(metaclass=_CDataMeta):
     _b_needsfree_: bool
     _objects: Mapping[Any, int] | None
     @classmethod
-    def from_buffer(cls: Type[_CT], source: _WritableBuffer, offset: int = ...) -> _CT: ...
+    def from_buffer(cls: Type[_CT], source: WriteableBuffer, offset: int = ...) -> _CT: ...
     @classmethod
-    def from_buffer_copy(cls: Type[_CT], source: _ReadOnlyBuffer, offset: int = ...) -> _CT: ...
+    def from_buffer_copy(cls: Type[_CT], source: ReadableBuffer, offset: int = ...) -> _CT: ...
     @classmethod
     def from_address(cls: Type[_CT], address: int) -> _CT: ...
     @classmethod