This option is similar to --no-history (avoid bloating the repository
size) but keep data. We squash all commits from gh-pages branch filtered
on the current commit message.

This allow workflows like keeping commits and data for a released doc
and always override nightlies documentations.

Add env var FILTER_BRANCH_SQUELCH_WARNING to avoid warnings and
temporisation when git-filter is called.

Avoid deprecated Python version 2.7, being removed from GitHub Actions
images.