Use PyPI package name for flufl-i18n instead of `flufl.i18n` to fix #1085
Due to the issues identified in #1085 from my deep diving into the codebase in a venv, it is necessary to change the requirements to match the PyPI package names. Otherwise, the system will fail with a "DistributionNotFound" error from the underlying packaging utils and mailman core will fail to launch under any circumstances.
This was tested in a venv on Ubuntu 22.04, and this change should be the only change needed to fix this issue.