[acdha]

I'm a bit mixed on a flag because C library dependencies are a fair support burden and an awful lot of people do not have workloads where a pure Python implementation is a significant performance issue because it's lost in the actual work done by the database.

I'd agree with something like a warning on first load or maybe something like an optional package so you could pin `psycopg3[cext]` if you want to guarantee the extension installed.