Python 3.6 is dead: now enter End-of-Life phase

[miurahr]

Python 3.6 is now EOL, in 2021 Dec. 23
All python users are recommended to upgrade to use Python 3.7 and later.

Current aqtinstall v2.0.5 marks minimum python version is 3.6.
It is good chance for me to increase it to python 3.7 in next few weeks.

python 3.11 will arrive in summer/fall of 2022. When reaching RC version, I'd like to test aqtinstall with it.