tox-run-command stopped working after upgrading to 3.21.0

[webknjaz]

FTR it worked with tox v3.20.1.

Here's how it tracebacks: https://github.com/cherrypy/cheroot/runs/1671994910?check_suite_focus=true#step:13:52.

It's supposedly broken by tox-dev/tox#1720.