return demo_sent_subjectivity instead of print

[andrewlin0]

I simply changed the output format of the demo_sent_subjectivity to return instead of print to avoid the program to have to run longer because it prints out each. result to the console.

[tomaarsen]

The reason the demo_* functions don't tend to return anything, is because they are not meant to be used much. In my opinion, these functions should not even be importable from NLTK. They are only meant to quickly and effectively show how functions/classes in that file can be used.
Because of that, I prefer printing over returning, as it helps with effectively displaying the output.

[tomaarsen]

#2903 has fixed a broken test case which snuck into this PR. I've merged develop into this PR to resolve this broken test.
That said, this PR still has broken tests from the PR itself. (pre-commit fails because of incorrect formatting, running black or pre-commit run --all-files should solve it)

[tomaarsen]

I'll be closing this.