Update MANIFEST.in.

[wRAR]

I noticed than tox -e twinecheck prints many warnings and this led to updating MANIFEST.in to the current project state. I couldn't add --strict to twine check though, because both prune docs/build and global-exclude __pycache__ *.py[cod] cause warnings about no files affected.

Some lines are removed because they are on by default, following https://setuptools.pypa.io/en/latest/userguide/miscellaneous.html. I also used https://pypi.org/project/check-sdist/ to see that the following files are omitted:

  .bandit.yml
  .bumpversion.cfg
  .coveragerc
  .flake8
  .git-blame-ignore-revs
  .gitattributes
  .gitignore
  .isort.cfg
  artwork/README.rst
  artwork/qlassik.zip
  artwork/scrapy-blog-logo.xcf
  artwork/scrapy-logo.jpg
  pylintrc
  sep/README.rst
  sep/sep-001.rst
  sep/sep-002.rst
  sep/sep-003.rst
  sep/sep-004.rst
  sep/sep-005.rst
  sep/sep-006.rst
  sep/sep-007.rst
  sep/sep-008.rst
  sep/sep-009.rst
  sep/sep-010.rst
  sep/sep-011.rst
  sep/sep-012.rst
  sep/sep-013.rst
  sep/sep-014.rst
  sep/sep-015.rst
  sep/sep-016.rst
  sep/sep-017.rst
  sep/sep-018.rst
  sep/sep-019.rst
  sep/sep-020.rst

which looks OK to me.

Not sure if we should add check-sdist to CI.

[wRAR]

(not sure why .github/ is not listed, I think it's not in the sdist tarball either)