Added tensorflow (<2) dependency in setup.py

[HitLuca]

Closes #221

[kousu]

This implicitly pins python<=3.6. Maybe this needs a second change to mark that?

$ python --version
Python 3.8.0
$ pip install 'tensorflow<2'
ERROR: Could not find a version that satisfies the requirement tensorflow<2 (from versions: 2.2.0rc1, 2.2.0rc2, 2.2.0rc3, 2.2.0rc4, 2.2.0, 2.3.0rc0, 2.3.0rc1, 2.3.0rc2, 2.3.0)
ERROR: No matching distribution found for tensorflow<2

[HitLuca]

I can confirm that we would need python to be pinned to <=3.6