ERROR: frontmatter 3.0.5 has requirement PyYAML==3.13, but you'll have pyyaml 5.1.1 which is incompatible

Dears,
during
bench switch-to-branch version-12 --upgrade
the following error is received:
ERROR: frontmatter 3.0.5 has requirement PyYAML==3.13, but you’ll have pyyaml 5.1.1 which is incompatible.
I have uninstalled pyyaml befor this upgrade just to let the upgrade script to do its thing! However, it looks like I have no control over this package relation and possibly the
bench update --patch
errors are because of this. Any idea?