Deprecate DataBuffer.`__len__`

When a DataBuffer is really large, len(DataBuffer) can cause Python to complain about overflow.

Although this is not easy to trigger now -- a large enough DataBuffer that triggers the issue will crash BN first. Since we are allocating a concrete buffer for the DataBuffer.

The issue names DataBuffer.len but does not identify a file or test. Locate that entry point, inspect how DataBuffer is exposed to Python, and search for related references or compatibility documentation. Done means the deprecation is represented consistently and relevant checks pass.