CP-41819: Start py3 migration with config for isort and apply isort

[bernhardkaindl]

This the the 1st PR to migrate to Python3:

Start off with a defined config and as most imports will have to be updated, support the imports to be placed using isort at the expected place.

Changes:

• Add .pyproject.toml with configuration for isort and line length
• Add setup.cfg, enable pip install . with setuptools_scm
• Apply an initial isort to prepare upcoming imports using isort.