Add __len__ to typing

arro3-core/python/arro3/core/_core.pyi

@@ -210,6 +210,7 @@ class Buffer:
     """An Arrow Buffer"""
     def __init__(self, buffer) -> None: ...
     def __buffer__(self, flags: int) -> memoryview: ...
+    def __len__(self) -> int: ...
     def as_bytes(self) -> bytes:
         """Copy this buffer into a Python `bytes` object."""