[MM3-users] Mailman installation results in KeyError: 'env_py'

Hi,

I'm trying to install Mailman 3 on Debian 9. However, it doesn't let me do.

$ virtualenv -p python3 env
[...]
$ . ./env/bin/activate
$ pip install mailman
[...]
Successfully installed [...]mailman-3.1.0[...]
$ mailman info
Traceback (most recent call last):
  File "/home/mailman/mailman-test/env/bin/mailman", line 11, in <module>
    sys.exit(main())
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/mailman/bin/mailman.py", line 97, in main
    initialize(config_path)
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/mailman/core/initialize.py", line 185, in initialize
    initialize_2(propagate_logs=propagate_logs)
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/mailman/core/initialize.py", line 156, in initialize_2
    config.db = getUtility(IDatabaseFactory, utility_name).create()
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/mailman/database/factory.py", line 55, in create
    SchemaManager(database).setup_database()
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/mailman/database/factory.py", line 96, in setup_database
    alembic.command.stamp(alembic_cfg, 'head')
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/alembic/command.py", line 504, in stamp
    script.run_env()
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/alembic/script/base.py", line 425, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/alembic/util/compat.py", line 83, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 673, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/mailman/database/alembic/env.py", line 38, in <module>
    @public
  File "/home/mailman/mailman-test/env/lib/python3.5/site-packages/public/public.py", line 15, in public
    else sys.modules[thing.__module__].__dict__)
KeyError: 'env_py'

I'm not a Python developer, I usually use Ruby for anything web-related, thus I can't make anything of this. Since the error message conveys exactly no information that I could use to find the problem and Google provides no mailman-specific information on it, I don't know what to do now. Any hint is appreciated.

Python version, if of any use:

$ python --version
Python 3.5.3

Greetings Marvin

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