feat: `NoConstraint`, a constraint for representing no constraint #59

	name: Type annotation (project)

	on:
	push:
	paths:
	- 'datalad_core/**.py'
	- '!/tests/.py'

	jobs:
	check-types-project:
	runs-on: ubuntu-latest
	steps:
	- name: Setup Python
	uses: actions/setup-python@v5
	with:
	# run on a "fresh" python, but see mypy flag to check for the oldest supported version
	python-version: 3.12
	architecture: x64
	- name: Checkout
	uses: actions/checkout@v4
	- name: Install hatch (which pull mypy)
	run: python -m pip install hatch
	- name: Type check project
	run: \|
	# get any type stubs that mypy thinks it needs
	hatch run types:mypy --install-types --non-interactive --follow-imports skip datalad_core
	# run mypy on the full project.
	# run on the oldest supported Python version
	hatch run types:mypy --python-version 3.9 --pretty --show-error-context datalad_core

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