gen-requirements: install python-discovery

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'

Looks like installing python-discovery make things work again.

Author: xrmx

gen-requirements.txt

@@ -7,3 +7,5 @@ requests
 tomli
 tomli_w
 hatch
+# for virtualenv 21 support
+python-discovery