lobe-python 0.3 refactor

[mbeissinger]

0.3 release

• More flexible and backwards-compatible with Lobe signature files. Checks for old signature constants and file layouts.
• tf_backend uses the tf 2.4 and saved model loader rather than the contrib inference library.
• Install the right version of tflite-runtime at setup.py install
• Remove double underscore variables to be more pythonic (and misc other style changes)
• Add ONNX backend