blubber: pin tox version in `verify-deps` variant
Blubber installs the latest available tox version by default. tox
4.9.0 is not compatible with chardet 5.1.0 used by Scap and was
causing verify-deps
to fail.
We don't want to tie a production dependency to the tox version used by blubber, so instead I've pinned the tox version (plus deps) to a compatible version in the variant