TF2.16 support: hermetic python shim, TF 2.16.2 pin, legacy Keras path, setup/packaging fixes

[psamanoelton]

Summary

• Enable building TFQ against TensorFlow 2.16.2 with Bazel 6.5.x
• Keep Keras 2 (“tf-keras”) path via TF_USE_LEGACY_KERAS=1 to avoid Keras 3 breakages
• Allow selecting system/venv/conda Python; wire that interpreter through Bazel
• Make wheel-build script honor chosen Python and include setuptools

Key changes

• WORKSPACE: bump @org_tensorflow to 2.16.2, add a tiny @python local_repository shim
• third_party/python_legacy/defs.bzl: export interpreter as a string (what TF’s local_config_python expects)
• configure.sh: --python= flag; generate .tf_configure.bazelrc + .bazelrc + the python shim
• .bazelrc: ensure TF_USE_LEGACY_KERAS=1 for build/test; keep existing warnings/rpath tweaks
• release/build_pip_package.sh: invoke ${PYTHON_BIN_PATH:-python3} and ensure setuptools present
• setup.py: (optionally) relax deps to run on TF 2.16 (sympy==1.14); extras pin tensorflow==2.16.0

Testing

• Linux/Ubuntu 22.04 (Docker), Python 3.11, Bazelisk → Bazel 6.5.0
• bazel build ... release:build_pip_package succeeds
• Unit tests (scripts/test_all.sh) pass with
• Wheel installs and imports with TF 2.16.2 + tf-keras 2.16.0, Cirq 1.3.0, NumPy 1.26.4

Notes

• This keeps TFQ on Keras 2 path; Keras 3 migration can be handled in a a future PR.
• This changes will help on the incoming upgrades to 2.20.

[mhucka]

Thank you for the work, and I'm sorry about the number of changes requested here …