strip ANSI escape codes from the output

[keewis]

pytest somewhat recently has started to also add colours to the diffs of built-in structures (like dict or list), which means we see the escape codes in the created issue (setting the CI env var usually disables colours, but at least in the projects I work on we set FORCE_COLORS because it makes reading the output a lot easier).

To avoid that, we strip the escape sequences here.

Ideally I'd want to have pytest-reportlog do this for us (after all, reportlog is designed to be machine-readable), but this would take a while to roll out.

[keewis]

I've created pytest-dev/pytest-reportlog#83 to hopefully fix this within either pytest-reportlog or pytest