community/py3-psycopg: remove typing-extensions from setup.cfg

Without this change, typing-extensions is still referenced in /usr/lib/python3.11/site-packages/psycopg-3.1.12.dist-info/METADATA and could cause a repeat of a00b4f54 / e13cc6da

This follows the tradition of bbe602ff which removes typing-extensions from pyproject.toml

Also, quiet shellcheck, as the lines it is complaining about are copied almost exactly from default_pyc() of /usr/bin/abuild