Avoid hitting network during test_virtualenv.test_test_command.

In test_test_command_install_requirements, uninstall setuptools to avoid getting some other version in the environment.

Replace incorrect statement about triggering installation of dependencies to indicate that the handling is mostly implementation specific. Ref pypa/setuptools#1471.

docs: recommend pypa build instead of pep517.build

Tests once again pass on an extract of the sdist.

Fix syntax in test_build_meta, version should not have quotes. Bug was masked by LegacyVersion parsing.

Adopt jaraco/skeleton

Fix tests with pytest 6.2

typo

The latest pytest deprecates pytest.yield_fixture in favor of
pytest.fixture [1]. The changelog [2] says that both are the same.

[1] https://github.com/pytest-dev/pytest/pull/7988
[2] https://docs.pytest.org/en/stable/changelog.html#pytest-6-2-0-2020-12-12

setuptools appends --find-links to pip if dependency_links is found, and
both takes URLs [1][2].

[1] https://github.com/pypa/setuptools/blob/v51.0.0/docs/userguide/dependency_management.rst#dependencies-that-arent-in-pypi
[2] https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-f

Remove tests guaranteeing that (vendored) dependencies can be installed without setuptools.