[3.12] gh-126499: test_ssl: Don't assume err.reason is a string (GH-126501) (GH-126573)

gh-126499: test_ssl: Don't assume err.reason is a string (GH-126501)

The skipping machinery called `getattr(err, "reason", "")` on an arbitrary
exception. As intermittent Buildbot failures show, sometimes it's set
to None.

Convert it to string for this specific check.
(cherry picked from commit 78ad7e632248dc989378cabeb797b9f3d940d9f2)

Co-authored-by: Petr Viktorin <encukou@gmail.com>