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 <[email protected]>

---------

Co-authored-by: Emídio Neto <[email protected]>

Author: xrmx

gen-requirements.txt

@@ -7,3 +7,5 @@ requests
 tomli
 tomli_w
 hatch
+# TODO: stick with virtualenv < 21 until a new hatch release
+virtualenv<21