[FEATURE REQUEST] Ability to read/write from/to BytesIO buffers in Python API

[iliyang]

Thank you for the amazing OpenEXR 3.3 release — it's so much more streamlined, faster, and makes life so much easier 👏

One important feature I'm missing is loading and saving EXRs from/to memory buffers. This is particularly relevant when processing large numbers of files that live on a remote storage. The Python API currently supports file-system input/output only, which necessitates dealing with temporary files. It would be so much better (and faster) to avoid that. My understanding is that the underlying C++ library already supports arbitrary stream input/output, so I hope exposing that to Python is not that complicated.

This feature was hinted as a possibility in #1758, so my fingers are crossed 🤞

Thanks a lot!