Fix tensorflow version compatibility matrix

[carlogrisetti]

Either we lower TensoFlow requirement to a compatible version for Python 3.6, or we drop Python 3.6 from the supported interpreter versions.

Although TensorFlow has been recently bumped up in version (and that's the cause of this issue) I would be in favor of dropping support for Python 3.6 (as it will go out of support in the near future anyways)