[3.10] gh-96861: Check for unset sys.executable during venv creation. (GH-96887) (GH-96918) (cherry picked from commit 2cd70ffb3fe22d778d0bb6ec220fdf67dccc1be6) Co-authored-by: Vinay Sajip <vinay_sajip@yahoo.co.uk> Co-authored-by: Vinay Sajip <vinay_sajip@yahoo.co.uk>