Python 3.9 compatibility

[Ana06]

scikit-learn 0.23 does not support Python 3.9: scikit-learn/scikit-learn#18621
Updating to 0.24 should solve the issue.

[williballenthin]

stringsifter uses pickle to serialize their model, so it may not be easy to simply update the version string:

https://github.com/mandiant/stringsifter/tree/master/stringsifter/model

[Ana06]

Even if scikit-learn 0.23 is not supposed to support Python 3.9, I have installed #32 using Python 3.9 and pip 20.1 and it seems to work. So we may not need to update scikit-learn to 0.24 to support Python 3.9 (still probably a good idea).

[ewalshmndt]

Fixed by #34