Can't bump tflite-micro: new "PIL" (Pillow) Python dependency

I tried bumping the tflite-micro submodule to current main, and encounter "No module named 'PIL'" when building software.

I don't think we have a precedent of using pip to install a missing Python package. I tried cloning https://github.com/python-pillow/Pillow under `third_party/python/`, but it was not found. It may be that the Pillow repository is structured a bit differently than other Python repos -- there is a PIL directory at `Pillow/src/PIL`.

Or perhaps we're currently not setting up PYTHONPATH for the tflite-micro build.