add python_requires to setup.py

[gfrlv]

Description

Make sure recent versions don't get installed with pip2. Analogous to dbcli/mycli#864

Checklist

• I've added this contribution to the CHANGELOG.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[j-bennet]

Well done!