Fix keras and tensorflow versions

[ste1ju]

Under the requirements, currently tensorflow 2.6 and keras 2.6 are used. This leads to the following error when executing the command from the basic tutorial for executing a code from a remote repository:

Traceback (most recent call last):
  File "webinar-0620/keras_mnist.py", line 48, in <module>
    y_train = keras.utils.to_categorical(y_train, args.num_classes)
AttributeError: module 'keras.utils' has no attribute 'to_categorical'

Fixing the versions to keras==2.4.0 and tensorflow==2.3.0 removes the issue, see here.