add db cleanup for PyPy

[mattip]

Claude helped me come up with this fix for hundreds of emitted warnings, with the justification:

SQLite reuses primary key IDs after a full table DELETE (it has no AUTOINCREMENT sequence to preserve), the next loop iteration inserts new Person and Address objects with the same IDs (1..N) as the previous run.

On CPython this is harmless: refcounting immediately collects the old objects when new_person is reassigned each iteration, clearing the identity map's weakrefs before the next loop. On PyPy, GC is deferred - the old objects remain alive, so the identity map still holds live entries for IDs 1..N. When the new objects are committed with the same keys, SQLAlchemy's _register_persistent detects the collision and emits:

SAWarning: Identity map already had an identity for (Person, (1,), None), replacing it with newly flushed object. Are there load operations occurring inside of an event handler within the flush?

This warning fires for every inserted row on every benchmark loop, flooding stderr and causing the benchmark to fail on PyPy.

Fix: call session.expunge_all() immediately after the bulk deletes to explicitly clear the stale session state before timing begins.

It would be nice to have someone who understands more about sqlalchemy than me review this for correctness, I am not sure this is the right place for a fix.

[zzzeek]

I think it would be a more realistic test to use a new session for each loop here

like

for loops in range(loops):
    with DBSession() as session:
         # everything else here

cheaper than calling expunge_all() and makes it clear where we're starting the operation

[zzzeek]

cool. does it work / fix the problem? this "with" syntax for the session was introduced probably long after this test suite was written

[mattip]

Yes, this fixes the warning/crash.