Add changelog and clean up Python package files

[fspreck-indiscale]

Summary

For both https://github.com/pangaea-data-publisher/pangaeapy/issues/51 and https://github.com/pangaea-data-publisher/pangaeapy/issues/50. This PR adds a changelog (empty, but for future releases), and tidies up the python package files a little s.th. a pip install . on top level works again for installing.

Most of the housekeeping files follow the structure that we (IndiScale) use for our own Python packages, see e.g., https://gitlab.com/linkahead/linkahead-crawler.

Changelog

This is empty yet, but it adds a changelog for future releases following Keep a Changelog. I highly encourage to use Semantic Versioning in the future, so I took the liberty of also stating that in the changelog file.

Package infrastructure

The former src/pangaeapy/setup.py has been moved to top level (together with the LICENSE.md) so that the structure adheres to https://setuptools.pypa.io/en/latest/userguide/quickstart.html. They also recommend to have the package configuration within a setup.cfg (or pyproject.toml) instead of the setup.py itself, so I moved the config accordingly.

Release helpers

I added a RELEASE guidelines document which can be adapted in the future with suggestions on how to do releases (again, based on the workflows we use for our own Python packages). I also added a release.sh script which takes care of pushing the package to PyPi.

Review and testing

• verify that the package can now be installed via pip+git directly by using my fork: pip install git+https://github.com/fspreck-indiscale/pangaeapy.git
• Most importantly, check that the new structure and documentation make sense to you.