libpq: SQLSTATE-based error matching for query failures

[gburd]

This is one of a small set of capability differences my pytest port [1] has that pytap/v3 lacks; opening them as separate PRs so each is reviewable on its own and you can pick what's worth taking. No obligation — purely to make the deltas visible.

What this adds

Your QueryError (and ResultData) currently carry only the error message text, so a test can't assert on a specific error condition except by string-matching the (locale-dependent) message. This adds:

• SQLSTATE extraction from the failed result via PQresultErrorField (already declared in your bindings.py, just unused) onto a new ResultData.sqlstate, carried on QueryError.sqlstate / .sqlstate_class.
• Named QueryError subclasses for the SQLSTATEs tests most often assert on — QueryCanceled, UniqueViolation, DeadlockDetected, SerializationFailure, etc. — dispatched by query_error_for() in query_safe.

So a test reads:

with pytest.raises(QueryCanceled):
    session.query_safe("SELECT pg_sleep(5)")   # under statement_timeout

instead of pytest.raises(QueryError) followed by a message regex. Every subclass is still catchable as QueryError/LibpqError, and an unmapped SQLSTATE still raises a plain QueryError, so nothing existing breaks.

Motivation / provenance

This revives the per-error-code idea from an earlier revision of Jelte's patchset (which you noted on -hackers had been dropped). SQLSTATE matching is the stable, locale-independent contract — message text changes with translations and across major versions, SQLSTATEs don't — so it's the right basis for negative-path assertions, of which the suite has many (constraint violations, lock timeouts, recovery-conflict cancellations).

The diff is deliberately small and in your idiom: +1 field on ResultData, the subclasses + a dispatch table in errors.py, and one call-site change in query_safe. query_oneval is left raising plain QueryError since it reads only the connection-level message (no per-result SQLSTATE); easy to extend later if wanted.

[1] gburd/postgres#24