Drop support for Python 3.6

[yuuki0xff]

Major desktop environments using Python 3.6 will EOL soon.

OS / Distribution	Python Version	EOL	Confirmed Date	References
Ubuntu 18.04	3.6	2023-03-31	2022-12-10	https://wiki.ubuntu.com/Releases https://packages.ubuntu.com/
Linux Mint 19	3.6	2023-03-31 based on Ubuntu 18.04	2022-12-10	https://en.wikipedia.org/wiki/Linux_Mint
Debian 10	3.7	2024-06-30	2022-12-10	https://wiki.debian.org/LTS https://packages.debian.org/
openSUSE 15.3	3.6, 3.9	2022-12-31	2022-12-10	https://en.wikipedia.org/wiki/OpenSUSE http://download.opensuse.org/distribution/leap/15.3/repo/oss/x86_64/
openSUSE 15.4	3.6, 3.9, 3.10	2023-11-30	2022-12-10	http://download.opensuse.org/distribution/leap/15.4/repo/oss/x86_64/
openSUSE Tumbleweed	3.8 ~	-	2022-12-10	http://download.opensuse.org/tumbleweed/repo/oss/x86_64/
Windows / official python build	3.7 ~	-	2022-12-10	https://devguide.python.org/versions/
Mac homebrew	3.7 ~	-	2022-12-10	https://formulae.brew.sh/formula/[email protected]#default
All platform / official python build	3.7	2023-06-27	2022-12-11	https://devguide.python.org/versions/