Import `keras` from the `tensorflow` namespace

```console
❯ rg keras
machine_learning/lstm/lstm_prediction.py_tf
9:from keras.layers import LSTM, Dense
10:from keras.models import Sequential
```

And thus, removing the `keras` dependency from `requirements.txt`.

Ref:
- https://github.com/keras-team/keras#under-construction
- https://keras.io/guides/functional_api/#setup