2 setup new repository and modernize tooling (#3)

* Replace setuptools with poetry

* Adapt renovate config and CI setup

* Move sources below src/

* Simplify CI testing

* Fix copilot code review warnings

* Update src/rest_flex_fields2/__init__.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/rest_flex_fields2/__init__.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* manage.py: Minor copilot formatting warnings

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>