Fix Buffer subclass type definitions

It does not make sense to special-case the `Packet` class concerning
buffer protocol support, we have multiple classes inhering our `Buffer`
class:

- `Packet`
- `Plane` and its subclasses `AudioPlane` and `VideoPlane`
- `SideData`

We cannot use `collection.abc.Buffer`, yet as this is only available in
Python 3.12 and up. We do however want to declare that `bytes(foo)` is
available, so declare a `__bytes__` method in `Buffer`.

av/buffer.pyi

@@ -1,4 +1,9 @@
+# When Python 3.12 becomes our lowest supported version, we could make this
+# class inherit `collections.abc.Buffer`.
+
 class Buffer:
     buffer_size: int
     buffer_ptr: int
     def update(self, input: bytes) -> None: ...
+    def __buffer__(self, flags: int) -> memoryview: ...
+    def __bytes__(self) -> bytes: ...

av/packet.pyi

@@ -1,8 +1,8 @@
-from collections.abc import Buffer
 from fractions import Fraction
 
 from av.subtitles.subtitle import SubtitleSet
 
+from .buffer import Buffer
 from .stream import Stream
 
 class Packet(Buffer):
@@ -22,4 +22,3 @@ class Packet(Buffer):
 
     def __init__(self, input: int | bytes | None = None) -> None: ...
     def decode(self) -> list[SubtitleSet]: ...
-    def __buffer__(self, arg1) -> memoryview: ...