Require virtualenv<21 for tox -e generate (#4265)

* gen-requirements: stay with virtualenv < 21

No idea what's going on but looks like hatch / hatchling has issues with
latest virtualenv 21:

Environment `hatch-build` is incompatible: module 'virtualenv.discovery.builtin' has no attribute 'propose_interpreters'

* Update gen-requirements.txt

Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>

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Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>