[3.13] gh-130070: Fix `exec(<string>, closure=<non-None>)` unexpected path (GH-130071) (#132627)

gh-130070: Fix `exec(<string>, closure=<non-None>)` unexpected path (#130071)

Fixed an assertion error (so, it could be reproduced only in builds with assertions enabled)
for `exec` when the `source` argument is a string and the `closure` argument is not `None`.

Co-authored-by: sobolevn <mail@sobolevn.me>
(cherry picked from commit 954b2cf031fb84ff3386251d5c45281f47229003)