Add Sixel Support

[theCapypara]

This adds support for displaying Sixels, in case VTE is compiled with support for it enabled. This is related to #1806.

Upstream issue for Sixel support: https://gitlab.gnome.org/GNOME/vte/-/issues/253

Side notes: While developing I came across three issues:

• 2156

• I needed to pin pylint to 2.13 because otherwise it would complain about options that no longer exist.
• I needed to pin Click to 8.0.4: https://github.com/psf/black/issues/2964

[Davidy22]

Sorry about the delay, I've dropped some older unsupported python versions from CI so it should run if you push a superficial change and I'll take a look in the meantime