Python 3.9 (#3926)

* Upgrade to Python 3.9

* pip install wheel for faster builds

* updating DIRECTORY.md

* requirements.txt: tensorflow; python_version < '3.9'

* keras requires tensorflow

* Rename lstm_prediction.py to lstm_prediction.py_tf

* Update requirements.txt

* updating DIRECTORY.md

* Update requirements.txt

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>