[MNT] move packaging to pyproject.toml

[fkiraly]

Moves packaging completely to pyproject.toml from multiple locations:

• setup.py
• requirements.txt
• poetry.lock

No changes to content except where uniformization is requred - e.g., matplotlib was inconsistently in the core and optional depset.

pyproject.toml is the current recommended packaging standard, see
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/

Also:

• removes poetry lockfile and related workflows for now.
• updates checkout and setup-python versions to latest