Recover from deadlock in session_truncate fixture

[jace]

This updated fixture does not have a test, so we must assume it works until a deadlock error happens again. My first attempt at writing a test did not go well because (a) we're testing for an error in a fixture, which requires pytest's pytester plugin (disabled by default) and a special operating condition for it, and (b) to simulate a PostgreSQL deadlock we need parallel database connections (thread, process or async with wait) that are also contained within a single test and teardown cleanly. This is a larger task than I had expected, so I'll leave it for another PR.