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>

Quotes in the name and packages field causes an "Invalid distribution name or version syntax" if you follow this quickstart guide directly. pep517.build also requires you to specify the directory.

Upgrade GHA actions using deprecated env mechanism

Refs:
* github.com/deadsnakes/issues/issues/135
* github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Removed quotes in setup.cfg which caused build to fail.

Switch to RTD v2 config

Reconstruct changelog with categories from history

Drop support for Python 3.5

📝 Update devguide links in the PR template

This is necessary because the document has been moved in PR #2426.

Fix sphinx strictness-related failures