Drop Python 2.7? about asttokens HOT 4 OPEN

Cross linking: #107

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@PeterJCLaw @dsagal

I just looked at Grist and their repo is setup with separate requirements.txt files for python 2 and python 3. As long as we don't change the public api, I think we will be fine with removing python 2 support.

Since the projects are supposedly tied, the way I suggest we go about it is:

1. bump asttokens to version 3 and remove support for python 2
2. release critical bug fixes to version 2.2.x if needed
3. update requirements.txt to include asttokens <=2.0.5,<3
4. update requirements3.txt to include the newer version 3

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Also, new grist installations use python 3.9 by default, so we can also drop 3.5 and 3.6 since they're dead.

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@aqeelat's suggested plan sounds good to me.

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