Qmk_firmware: [Proposal] Bump QMK CLI minimum version from 3.5 > 3.6

[Curry]

Proposed by @Erovia : Updating our minimum python version from 3.5 to 3.6 would allow us to take advantage of:

• F-Strings: PEP 498
• PathLib: PEP 428, Specifically in 3.6, will allow us to walk directories and retrieve all sub-folders and their files using pathlib PEP 519

Others?

To carry this out, we need to know the minimum supported versions of python 3 on all our supported platforms, going back to the oldest LTS version. Here is a table of our supported distributions, and the minimum available python version:

| Platform | Python Version |
| ------------- |-------------|
| Windows | 3.8.1 (MSYS) |
| MacOS | 3.7.6 (Homebrew) |
| FreeBSD | 3.7.6 |
| WSL | ? |
| Arch | 3.8.1 (Arch Packages) |
| Manjaro | 3.8.1 (Arch Packages) |
| Fedora | 3.7+ (31), 3.7+ (30) |
| Ubuntu | 3.5.1 (16.04 LTS), 3.6.5 (18.04 LTS) |
| Debian | 3.7.3 (10), 3.5.3 (9), 3.4.2 (8) |
| Gentoo | 3.6.9 (Note: Install script pins to 3.5, version no longer available) |
| Sabayon | 3.6.9 (dev-lang/python, no install version specified) |
| openSUSE Tumbleweed | 3.7.3 (Official) |
| openSUSE Leap 15.1 | 3.6.9 (Official) |
| Slackware | 3.8.1 (Slackware current) |
| Solus | 3.6.9 |
| Void | 3.8.1 |

This is the ones I could publicly find, if anyone is running one of these distributions that have a question mark, please comment it below.

As of right now, Ubuntu 16.04 LTS Xenial, Debian 8, and Debian 9 has <3.6 python support.
Additionally, the qmkfm/base_container python version is using Debian 9.

[Erovia]

Both Fedora 30 and 31 has 3.7+.

[Erovia]

It seems FreeBSD also has 3.7+.

[Erovia]

Solus is currently on 3.6.9 with plans to 3.7+.

[Erovia]

Sabayon: 3.6.9

[Erovia]

For WSL I believe it depends on the installed userspace, but I'm not a Win user.

[stale[bot]]

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[zvecr]

Completed in #8835