Bit objects not supported with Psycopg 2 and pg8000 #169

Hi, pgvector.Bit works as a query parameter with Psycopg 3 (it registers a dumper), but not with Psycopg 2 or pg8000:

cur.execute('SELECT %s::bit(3)', (Bit('101'),))       # psycopg2: ProgrammingError: can't adapt type 'Bit'
conn.run('SELECT CAST(:b AS bit(3))', b=Bit('101'))   # pg8000: "i" is not a valid binary digit

Passing to_text() works, but it's inconsistent with Vector/HalfVector/SparseVector, which both drivers adapt. The pg8000 case is a bit worse since it sends the wrong value rather than raising a clear error.

For both drivers this is a small text adapter (same pattern as HalfvecAdapter / register_out_adapter); happy to open a PR if you'd like. (asyncpg left out on purpose, per #110.)

Tested with main (b5d9b70), Postgres 17, pgvector master.