Upgrade packaging setup

The current setup for packaging is a bit dated, we use setup.py and don't use nide features like extras etc. What we could achieve:

• Use pyproject.toml for configuring the whole package
• Use extras to install optional packages: pip install mollie-api-python[examples] would install flask, mollie-api-python[test] would install pytest etc. This is better than using separate requirement.txt files, or write things in documentation.
• We could setup minimum and maximum versions for dependencies, ensuring that we automatically upgrade a dependency for which a vulnerability was found to a newer version (or trigger a conflict for the vulnerable version).
• Minimum/maximum version specifiers for dependencies can also be used by @dependabot to inform us automatically about vulnerabilities we're not aware of.
• Specify supported python versions for the package, helping our users to use the correct package for their environment.

Reference docs: https://packaging.python.org/en/latest/tutorials/packaging-projects/

Start by reviewing the existing setup.py and the referenced Python packaging guide, then map the current requirement files and supported Python versions. The work is done when packaging is configured through pyproject.toml with optional extras, dependency version bounds, and declared Python support, while the package can still be installed successfully.