Drop Python 3.6 support

[jiasli]

Description

Python 3.6 is reaching its end of support on 2021-12-23: https://www.python.org/downloads/

Maintaining code which can work perfectly on Python 3.6 and above versions is a maintenance burden:

• {Packaging} Drop mock library #19024 (comment)

We need to drop Python 3.6 like how we dropped Python 2.7:

• Drop support for Python 2 in azure-cli #11362
• [Packaging] BREAKING CHANGE: Drop support for Python 2.7 #11363

[yonzhan]

Drop Python 3.6

[jiasli]

One big problem with CentOS 7 (copied from #20608 (comment)):

• Python 3.6 has been deprecated on 2021-12-23 (https://www.python.org/downloads/), but CentOS 7 currently only has Python 3.6.
• CentOS 8 has been deprecated on 2021-12-31, leaving the old CentOS 7 be the only supported stable distribution (https://wiki.centos.org/About/Product).

Also see:

• https://community.home-assistant.io/t/python-support-on-centos-after-version-3-6-deprecation/146830
• https://thehackernews.com/2021/09/moving-forward-after-centos-8-eol.html
• https://forums.centos.org/viewtopic.php?t=78236&p=328761

[jiasli]

Good news is that Red Hat Enterprise Linux (RHEL) 8 now has Python 3.6, 3.8, and 3.9:

• https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/assembly_installing-and-using-python_configuring-basic-system-settings
• https://developers.redhat.com/blog/2019/05/07/what-no-python-in-red-hat-enterprise-linux-8