Use pypoetry for package management (& add flake8 config)

[FichteFoll]

• Replace requirements.txt.
• Requires Python 3.8 via assignment expression used in client.py, determined using vermin.
• Move frontend folder into the warp_journal package.
• Define a warp-journal script.

Note: I don't know how/whether this affects packaging with the tool you're using for building an .exe because I only use Linux. Note2: I added exclusions for the flake8 codes that are clearly & intentionally violated by the code as-is.