Transparently support running under CPython and PyPy.

`psycopg2` does not work under PyPy; the `psycopg2cffi` package rectifies this by using `cffi` to bind to PostgreSQL's `libpq`. This change will allow applications using bowab to work under either interpreters.

It might be a cleaner option overall to not declare a dependency on `psycopg2` at all, however. There is no code in bowab that imports from `psycopg2` directly. This could impact other packages that depend on bowab that haven't correctly specified a dependency, however.