Thanks, Steven. I did run
pip install wheel additionally.
Then I uninstalled the mailman packages, via running
pip uninstall wheel mailman psycopg2-binary\<2.9 to be able to begin anew.
Now I run
pip install wheel mailman psycopg2-binary\<2.9
It runs through but still prints out this one error:
ERROR: Command errored out with exit status 1: command: /opt/mailman/venv/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-rnm7lm08/mailman/setup.py'"'"'; __file__='"'"'/tmp/pip-install-rnm7lm08/mailman/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d /tmp/pip-wheel-jky3c7_5 cwd: /tmp/pip-install-rnm7lm08/mailman/ Complete output (6 lines): usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...] or: setup.py --help [cmd1 cmd2 ...] or: setup.py --help-commands or: setup.py cmd --help
error: invalid command 'bdist_wheel'
ERROR: Failed building wheel for mailman