refactor(build): change setup.py to pyproject.toml and supress cython warning

[hassec]

I was motivated to fix #289, but when starting to work on the old setup.py I was curious if you would be open to a bit of a modernisation, see below:

Changes in this PR:

• Move package/build configuration into a pyproject.toml following the adopted python standards.
• Only have a very minimal setup.py that translates omas_cython.pyx to omas_cython.c at the time the sdist package is created. This has the benefit that Cython is no longer a dependency for users. They *.c file still gets compiled during package install on the users computer, but that still is pretty quick and doesn't produce any warnings.

This PR also removes the need to manually traverse the file hierarchy to create packages and package_data lists. This is now automatically handled.

Editable installs still work by simply running:

pip install -e .

To build a sdist package using build run:

python -m build --sdist

from the root of the project. This creates an isolated virtual environment to build the sdist according to the spec in pyproject.toml.

[hassec]

With this change the compiled version of omas_cython should be available to everyone. Also windows users.

[hassec]

This automatically sets the version to what is found in omas/version

[orso82]

We're certainly very open!!! Thank you @hassec for taking a deep dive into the setup files of OMAS!!

Perhaps @kalling can comment here. I have not kept up to date with the latest and greatest ways to setup a Python package.

[hassec]

It would require bumping this from 3.6 to 3.8.
3.6 was end of life in 2021, so I hope that this is ok?

[smithsp]

OMFIT is at python=3.7 and uses omas as an integral part. What changes are you making here that require python>=3.8?

[hassec]

I looked at what the recent versions of setuptools require.
But what would be needed here is support for PEP-621 which looks like it landed in setuptools version v61-0-0 which still supports >=3.7.

I could try and see giving if it works for 3.7.

[hassec]

@smithsp I double checked and I was correct that in theory 3.7 should have been sufficient to enable usage of pyproject.toml based package configurations.

However, the problem here is the dependency on pygacode which is forced to version 0.71 from 2022 because that's the last version that supports 3.7, and that version does not correctly specify its build dependencies (requires numpy). This is fixed for versions >1.0 but those require python >=3.8 which is why the build here works for 3.8 but nor for 3.7.

[smithsp]

We are actively working on migrating OMFIT to a later python version.

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