Vendor a copy of tomli in easybuild.tools.tomllib

[Flamefire]

Include https://github.com/hukkin/tomli/tree/2.3.0 in easybuild as easybuild.tools.tomllib which will be replace by tomllib in Python 3.11+

Slightly patched to make it compatible with Python 3.6:

• Type hints (e.g. tuple[int, int] -> tuple)
• /, * in function params (end of positional args)

I copied the example from toml.io which (hopefully) triggers most code parts of the library to shake out any issues.

I decided for tomli instead of extracting tomllib from CPython because of its goal:

A version of Tomli, the tomllib module, was added to the standard library in Python 3.11 via PEP 680. Tomli continues to provide a backport on PyPI for Python versions where the standard library module is not available and that have not yet reached their end-of-life.

It currently doesn't support Python 3.6 but 3.8 so we can use it when dropping Python < 3.9

Requires:

• Switch to setuptools and find_packages #5065 (simplify with find_packages to avoid listing the new ones)
• Add back CI for Python 3.6 by using pyenv #5064 (test with Python 3.6 to ensure compatibility)

[boegel]

@Flamefire Can you clarify why this requires switching to setuptools (#5065)?
I'm not keen on merging that, we've been bitten in the ass by setuptools on too many times in the (distant) past...

[Flamefire]

Without setuptools you'd need to list all "packages" in the setup call and it took me multiple tries to figure out which it actually requires.

What were those issues and are they still relevant? Our usage here might have changed enough that we are "standard" enough not to run into them (anymore). Especially if those issues were from the Python 2 days. Seems most advise using setuptools unconditionally (if not even pyproject.toml) for non-ancient Pythons

An alternative if you really want to play it safe:

• Import setuptools.find_packages if possible
• check output against our manual list and error on difference printing the correct list
• Have EASYBUILD_IGNORE_FINDPACKAGES=1 to skip that as a way out if it unexpectedly causes issues on some system.

But I'd really like to avoid error-prone adding (more) things to a hard-coded list if possible.

[boegel]

We should be discussing this in #5065, so I'm moving the discussion there...

Ideally, this PR is reworked so the changes in #5065 are not required at all, since I'm really reluctant to make setuptools a hard requirement for EasyBuild where we can avoid it.

[boegel]

lgtm