Introduce requirements.txt for CI and easier Python (venv) setup

Summary

Introduces a requirements.txt file. This file can be used by the CI and by users to set up the Python environment more easily.

Related Issues

• Related to #207 (closed)

Checklist

• Documentation updated, if necessary
• Update version info, if changes are worth mentioning (Develop section in doc/src/version_info.dox, add section if not yet included)