Add buildpack for pyproject.toml to configure container image

[rgaiacs]

Related to #1427

This is not yet ready for review!

The code is based on the Pipfile buildpack. We probably want to refactor some portions to avoid code duplication.

[rgaiacs]

577cb1b implements a minimal working version. It was tested with https://github.com/rgaiacs/binder-examples-pyproject.

Required changes before merge

• To parse the pyproject.toml, we require tomllib that is part of Python 3.11 standard library. This means that we will have to change the minimal version of Python required by repo2docker to Python 3.11 or later.

Changes that can be done in another pull request

• Document the use of pyproject.toml
• Migrate https://github.com/rgaiacs/binder-examples-pyproject to https://github.com/binder-examples/pyproject

[minrk]

Can we do:

Suggested change

	"""PATH="${KERNEL_PYTHON_PREFIX}/bin:$PATH" \\
	pip install --no-cache-dir --editable .
	""",
	"${KERNEL_PYTHON_PREFIX}/bin/python3 -m pip install --no-cache-dir --editable .",

to be even more explicit than relying on $PATH?

[rgaiacs]

All tests are passing after support to Python < 3.11 was dropped.

[manics]

We probably want to refactor some portions to avoid code duplication.

I've got a PR that refactors the handling of runtime.txt
#1428

[rgaiacs]

I rebased this pull request and I made minor changes to reduce the code duplication. It need to be reviewed again.