Enable PEP-621 style build meta information

[TonyXiang8787]

Currently the build meta information is stored in many files, namely

• setup.py
• setup.cfg
• pyproject.toml
• requirements.txt
• test-requirements.txt

PEP-621 allows to store all the project meta-data in one single file pyproject.toml. This should be the preferred way. Maybe we can add a separate VERSION file to ease the version bump script.