Move to tensorflow>=2.0.0

[yoptar]

We'll have to remove all tf.contrib usage and move from bert_db to transformers everywhere.

[Barracuda72]

To me this looks like a major problem, as TensorFlow 1.x doesn't support Python 3.8+, and not going to:

tensorflow/tensorflow#39768

So Py3.8 users (running Ubuntu 20.04, e.g.) can't use most (if any at all) of DeepPavlov models.
(And it's better to be straight dead than to mix different Python versions on one machine.)

[andreabrduque]

Are there any updates on this issue?

[IgnatovFedor]

Tensorflow components will be removed in 1.0.0. In 1.0.0rc0 all pytorch-based models support python 3.6-3.9.