Re-export names from psycopg3.types

Re-export names from psycopg3.types

This is useful to silent mypy errors when implicit reexport is disabled
(e.g. in strict mode) like:

    error: Module 'psycopg3.types' does not explicitly export attribute
    'Jsonb'; implicit reexport disabled  [attr-defined]

https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport

Accordingly, we import Int4 from psycopg3.types directly in
test_copy.py.