Update packaging guide

[greschd]

Remove guide regarding setup.py, and recommend flit with pyproject.toml instead. For applications (which should have pinned versions instead of loose ones), recommend poetry.

[greschd]

@akaszynski I noticed e.g. in "Development Practices" there are some places referring to

pip install -r requirements_test.txt

Since the template now uses optional requirements for this it might be better to change to pip install -e .[test]. However, in general this depends on how exactly the repository being worked on is structured.

[akaszynski]

Agree with the changes and appreciate the discussion of the merits of filt vs poetry.

[akaszynski]

@greschd, let's modify this to reflect our choice in poetry.

[MaxJPRey]

LGTM.

outdated

[akaszynski]

Pinging @jorgepiloto to update this based on ansys/ansys-templates#2

[akaszynski]

Thanks for your work @greschd!