[3.15] gh-154303: Skip test_sqlite3 CLI completion tests on OpenBSD (GH-154304) ...

[3.15] gh-154303: Skip test_sqlite3 CLI completion tests on OpenBSD (GH-154304) (GH-154305)

OpenBSD's readline does not honor "completion-query-items 0", so completing
many candidates blocks on a "Display all N?" prompt that hangs the
pseudo-terminal opened by run_pty().  Skip the whole Completion class on
OpenBSD, like the existing libedit skip.
(cherry picked from commit 63b564fc8728036f165805407f7d2b4e8bfd3ef0)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>