check/STAT/gf_axisregistry: Allow Italic AxisValues in slnt axis

[felipesanches]

This is a followup to PR #4215 that was closed due to too many months of inactivity.

The code was available as 3 commits at this branch, by @m4rc1e: https://github.com/m4rc1e/fontbakery/tree/user-italic2

To be sure nothing is lost, I have also generated a patch-set with git format patch -n3. Those 3 patches are attached to this message.

For additional context, please read the description of the old PR at https://github.com/fonttools/fontbakery/pull/4215

patch-set: fontbakery-pull-4215-patch-set.zip