Update supported python versions in package metadata

[cob16]

There are a few compounding factors on what python versions work with this repo

• Jinja templates now require python versions >=3.7
• current boto version will prevent usage of this package with python 3.10 due the removal of deprecated stdlib functions ( moving to boto3 would solve this)

Looking at download statistics it looks like most if not all users are using versions later than python 3.7
https://pypistats.org/packages/cfstacks

Looking at this I think it would be good to update this package setup.py to suggest that python versions >= 3.7 and <3.10 are supported better reflect the current status quo

Putting this as an issue rather than a PR in the hope it will foster some discussion