[3.14] gh-135773: have pyvenv.cfg without home key anchor a venv and deduce home (GH-135831) (#136287)

gh-135773: have pyvenv.cfg without home key anchor a venv and deduce home (GH-135831)

This is still formally undefined behaviour, but we may as well
keep the *same* undefined behaviour as previous versions.

PEP 796 proposes a cleaner and more consistent replacement for 3.15+
(cherry picked from commit 93263d43141a81d369adfcddf325f9a54cb5766d)

Co-authored-by: Richard Levasseur <rlevasseur@google.com>