Drop python 3.6 support

[tokuhirom]

https://pythonspeed.com/articles/stop-using-python-3.6/?fbclid=IwAR2jGWnfJk-WTS61kbaJTwbiONMN0BgvAiMrcMrmW9yvVvSIVI0F-rlK-3s

It becomes EOL in December 2021.

[corona10]

FYI, https://devguide.python.org/devcycle/
There is the official list of EOLs for each version.
(I expect that you may already know)