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/

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

Resurrect docs interlinking

🔥 Get rid of the unused sphinx `Makefile`