Steps to reproduce on 12.3-RELEASE:

# pkg install py38-mailman
# service mailman onestart
/usr/local/etc/rc.d/mailman: WARNING: /usr/local/mailman/etc/mailman.cfg is not readable.
# service mailman onestop
Traceback (most recent call last):
  File "/usr/local/bin/mailman", line 33, in <module>
    sys.exit(load_entry_point('mailman==3.3.1', 'console_scripts', 'mailman')())
  File "/usr/local/bin/mailman", line 25, in importlib_load_entry_point
    return next(matches).load()
  File "/usr/local/lib/python3.8/importlib/metadata.py", line 77, in load
    module = import_module(match.group('module'))
  File "/usr/local/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 843, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/usr/local/lib/python3.8/site-packages/mailman/bin/mailman.py", line 22, in <module>
    from mailman.config import config
  File "/usr/local/lib/python3.8/site-packages/mailman/config/__init__.py", line 20, in <module>
    from mailman.config.config import Configuration
  File "/usr/local/lib/python3.8/site-packages/mailman/config/config.py", line 26, in <module>
    from flufl.lock import Lock
import os
  File "/usr/local/lib/python3.8/site-packages/flufl/lock/__init__.py", line 3, in <module>
    from flufl.lock._lockfile import (
  File "/usr/local/lib/python3.8/site-packages/flufl/lock/_lockfile.py", line 16, in <module>
    from psutil import pid_exists
ModuleNotFoundError: No module named 'psutil'

Do we miss some dependencies?

# pkg info --dependencies py38-flufl.lock-6.0
py38-flufl.lock-6.0:
	python38-3.8.12
	py38-setuptools-57.0.0
	py38-atpublic-3.0