Use psycopg2cffi module instead of psycopg2 if running on pypy

The psycopg2 package isn't available for PyPy.  This commit adds a check
if the runtime is PyPy, and if it is uses psycopg2cffi module in favor
of psycopg2. This is almost a drop-in replacement, except for one place
where an additional cast to string is required.