Fix SM compilation error caused by newer virtualenv versions.

Install a version known to work. Up-to-date 3.7, 3.8, 3.9 and 3.10 are confirmed to work, though versions shipped with older mac systems might not.

Reported by: Langbart

Tested By: Langbart

Fixes #6447

Differential Revision: https://code.wildfiregames.com/D4542