Pinned dependency conflict

[MrAlecJohnson]

trafficstars

The dependencies have some conflicts, starting with Scipy 1.4.1 and Pyarrow 0.16.0

NLTK also seems to be a version old enough to have security vulnerabilities.

Both of these should be fixable, but quite a few of the dependencies have had a lot of updates since this was made, so it might be worth doing a full check and update of requirements.