Use virtualenv instead of user pip

[johanvdw]

I think it is cleaner to use virtualenv instead of a pip --user:

• different instances use their own environment, which is nice in case of upgrades or plugin installs
• at least in debian, pip is actively telling you not to use it, unless you specify --break-system-packages

error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.
    
    If you wish to install a non-Debian-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
    sure you have python3-full installed.
    
    If you wish to install a non-Debian packaged Python application,
    it may be easiest to use pipx install xyz, which will manage a
    virtual environment for you. Make sure you have pipx installed.
    
    See /usr/share/doc/python3.11/README.venv for more information.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.

I'm opening as a draft PR because this will break arch compatibility.

[rixx]

Thank you! I added a few comments. Additionally, I may be blind, but where are you setting pretalx_python?

[rixx]

You've removed the parametrisation (%i) in this service file, fundamentally changing how it's used. I don't think that's a good idea – while I personally don't run this ansible role anymore (in favour of something already using venvs, albeit more customized ones), changing the behaviour like that doesn't seem great.

[rixx]

Included this in a separate commit, just to get it merged while we talk about the rest here.

[rixx]

You're using |default(omit) in some places but not in others, is that intentional?

[rixx]

Hmm, I don't think this is quite correct. Instead, we can just replace the collectstatic and compress commands with rebuild – rebuild runs collectstatic, compress and also compilemessages.

[johanvdw]

I should probably have been clearer: I wanted to get some feedback about the idea of using virtualenvs in the first place, which is why I opened this PR as a draft in the first place. Will rework it a bit (and split the npm part) and open this again.

[rixx]

Yeah, absolutely in favour of using venvs in general, thank you for looking into this! Didn't mean to reject the PR entirely, just the implementation details :)