memoryview: re-add inheritance from Sequence, set index and count to None (#12800)

This reverts commit f625e92.

Author: JelleZijlstra

stdlib/@tests/test_cases/builtins/check_memoryview.py

@@ -56,3 +56,6 @@
 # An invalid literal should raise an error.
 mv = memoryview(b"abc")
 mv.cast("abc")  # type: ignore
+
+mv.index(42)  # type: ignore
+mv.count(42)  # type: ignore

stdlib/builtins.pyi

@@ -834,7 +834,7 @@ _IntegerFormats: TypeAlias = Literal[
 ]
 
 @final
-class memoryview(Generic[_I]):
+class memoryview(Sequence[_I]):
     @property
     def format(self) -> str: ...
     @property
@@ -897,6 +897,11 @@ class memoryview(Generic[_I]):
     def __buffer__(self, flags: int, /) -> memoryview: ...
     def __release_buffer__(self, buffer: memoryview, /) -> None: ...
 
+    # These are inherited from the Sequence ABC, but don't actually exist on memoryview.
+    # See https://github.com/python/cpython/issues/125420
+    index: ClassVar[None]  # type: ignore[assignment]
+    count: ClassVar[None]  # type: ignore[assignment]
+
 @final
 class bool(int):
     def __new__(cls, o: object = ..., /) -> Self: ...