Documentation: generate changelog from GH releases

[penguinolog]

• Since 2.2.0 GitHub releases are used. Changelog in generated from merged PRs during release creation.
• Changelog for documentation is generated with: Link to release Link to PyPI

NOTE: Changelog can not be generated from forks
(no token assess by security reasons)

NOTE: On local run changelog will be not generated without token (SPHINX_GITHUB_CHANGELOG_TOKEN env var)

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[penguinolog]

See generated documentation in https://github.com/urwid/urwid/suites/16675041475/artifacts/952850345

[wardi]

something weird going on with the expand/contract thing on the sidebar, but maybe we could just remove that if it's too much trouble to fix. Otherwise looks good

[penguinolog]

@wardi technically I see issue with template rendering after sphinx update. Need sphinx theming guru to fix it. At current moment I can only open issue for it.

[coveralls]

Pull Request Test Coverage Report for Build 6348991037

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.38%

---

Totals
Change from base Build 6341349895:	0.0%
Covered Lines:	7514
Relevant Lines:	10286

---

💛 - Coveralls