Tests fail because librt does not support pypy

[2bndy5]

I noticed the tests in CI started failing on pypy builds because the the requirement-tests.txt specifies mypy:

pygit2/requirements-test.txt

Line 3 in a4ec27e

mypy

Which transitively requires librt. But librt cannot be installed for pypy. See mypyc/librt#16

The recommended fix (per librt maintainers) is to not use mypy on pypy.

mypy ; platform_python_implementation != 'PyPy'"

Or just don't require mypy if no unit tests require it installed.

Or if the unit tests do require mypy installed, simply skip running tests on pypy wheels in CI (using cibuildwheel test-skip settings in project.toml):

pygit2/pyproject.toml

Line 18 in a4ec27e

test-skip = "*-*linux_ppc64le"

would (I think) get turn into

test-skip = "*-*linux_ppc64le pp3*"

[2bndy5]

It looks like mypy was added as a test dependency in #1386. However, it does not actually require mypy for unit testing. It's possible that mypy should go into a requirements-dev.txt because mypy is more of a development-specific tool (not unit testing dependency).

I would also lump cibuildwheel into the new requirements-dev.txt so dependabot can easily auto update the pinned version of cibuildwheel (when you approve). However, the following dependabot config would have to be uncommented if pinning cibuildwheel in a requirements-dev.txt file:

pygit2/.github/dependabot.yml

Lines 18 to 25 in a4ec27e

	# - package-ecosystem: pip
	# directory: .github/
	# schedule:
	# interval: "monthly"
	# groups:
	# pip:
	# patterns:
	# - "*"

[2bndy5]

pygit2/build.sh

Lines 271 to 272 in a4ec27e

	$PREFIX/bin/pip install -r requirements-test.txt
	$PREFIX/bin/mypy pygit2 test

This (patch from #1387) explains the confusion between dev tool and test dependency. Running mypy against the unit testing sources also requires the test dependencies installed.