Requirements.txt allows an unsupported version of tf+keras

[zeroby0]

Describe the bug

The current requirements.txt allows for installation of Tensorflow and Keras versions that are not supported.

The latest release (https://github.com/tensorflow/model-optimization/releases/tag/v0.8.0) needs Keras 2.x, which leads to bugs like https://github.com/tensorflow/model-optimization/issues/1140

Please change requirements.txt to only allow keras upto 2.15 and create a new point release. I'll create a PR for this later if the authors are interested in merging.