(25 Apr 2023, @ankostis): REVIVE project, Bump DEPS

Fix all TCs and warnings for recent dependencies.

• FIX(SITE): bump to latest sphinx:

  • FIX(TCs): bumped jinja2, added MarkupSafe and unpinned pytest
    so that all TCs run OK again with latest deps.
  • fix(sphinxext.TC) probably graphviz generates slightly different captions in SVGs.
  • fix: version-parsing in docs/conf.py neglected since v1.0 and
    manifested now as erroneously parsing pep440-version id - simplified it a lot;
    stop using forgotten packaging dep.
  • fix: sphinx-extlinks >= 4 plugin demands %s also in caption or else
    pytest-fails, and a warning is issued on site generation.
  • fix(site) docutils-0.17+ broke long substitutions like |version|;
    fixed it with a docutils.conf file (instead of resorting to pinning).

• fix(build): unpin black==20.8b1 since it's been years since black was released
  and pip install -e[all] was failing when trying to build end-of-lifed typed-ast
  e.g. on Python3.11.

• style: black & isort imports all over (with a black profile for the latter).

• DOC: various nitpicks and diagram beauties.

• DOC: add section about diffs with schedula project.

• plot: various minor improvements.

• ENH(plot) add RC-Shape badges in data-nodes

• CHORE: TravisCI site lost :-( moved to GitHub Actions.