Compatible package versions are not provided in requirements.txt

[shervinea]

Context. Issue opened based on @w-t-2017's comment https://github.com/shervinea/enzynet/issues/4#issuecomment-824681634.

Background. This package relies on ever-changing dependencies listed as requirements such as Keras or Tensorflow. It would be useful to know which version works with enzynet's code to avoid any ad-hoc headache.

Current behavior. requirement.txt does not display compatible versions information.

Expected behavior. requirement.txt indicates which versions of the dependencies are certified to work with the package.