Update to modern Python project setup (setup.py to pyproject.toml)

[konstntokas]

This PR updates the setup of xcube using pyproject.toml instead of setup.py. I tested new setup by installing it locally and by uploading it to TestPyPI and installing it with pip install -i https://test.pypi.org/simple/ xcube-core==1.6.0.dev2

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[konstntokas]

In pyproject.toml, the name field cannot be dynamic (like e.g. "version").
I set the name to "xcube-core" since this will be name shown on PyPI.
The import runs as usual with import xcube. Some disccusion is given here

Will this name field in pyproject.toml have any implication with conda releases?

[pont-us]

@konstntokas The name shouldn't have any effect on conda-forge: conda-forge's own package name (or names) is defined explicitly in meta.yaml, and our feedstock configuration fetches the xcube source straight from GitHub so it doesn't care about the PyPI name.

[pont-us]

One minor change request, otherwise fine :)

[pont-us]

@konstntokas The name shouldn't have any effect on conda-forge: conda-forge's own package name (or names) is defined explicitly in meta.yaml, and our feedstock configuration fetches the xcube source straight from GitHub so it doesn't care about the PyPI name.

[forman]

Great!