Fixes syntax issues in quickstart (#2448)

Update quickstart.rst

s/517/PEP 517/

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

This was caused by #2399 CI invocation happening before
merging PR #2427 that made Sphinx nitpicky.

This change corrects the typo introduced by #2426.

Group development related files together

Add Python 3.9 trove classifier to the metadata

GHA: Replace 3.9 beta with 3.9 final

Link to build_meta documentation

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

Fix ReST syntax error in the PEP 517 link

docs: code-block style fix

docs: editorial word fix

Report test results in the AppVeyor UI

📝 Enable showing news categories in towncrier log