00443: gh-124651: Quote template strings in venv activation scripts

[frenzymadness]

(cherry picked from 3.9)

Patch backported from python#126301 by @vstinner

Victor, you have most of the context for a review of my backport to 3.6.

[frenzymadness]

I have a reproducer for the problem based on the original report. The original report is:

envname='";uname -a;"'
mkdir "$envname"
cd "$envname"
python3 -m venv .
. ./bin/activate

This patch fixes the problem.

Scratch build with the patch: https://koji.fedoraproject.org/koji/taskinfo?taskID=125509295

[vstinner]

LGTM