This makes sure that flake8's defaults are in use. Specifically, it
excludes `.tox/` dir that is known to contain a lot of files in nested
folders that are not supposed to be linted.

Refs:
* https://github.com/pypa/setuptools/issues/2501#issuecomment-749144396
* https://github.com/pypa/setuptools/pull/2486/files#r546877674
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-extend-exclude
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-exclude

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.

In a PEP 517 world without distutils, packagers will need to find another way to build and supply the dependencies

fix broken link to entry-points details

Fix all deprecated set-env and add-path uses @ GHA

Fixes syntax issues in quickstart (#2448)

Update quickstart.rst

s/517/PEP 517/

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>