[Cleaned up #37] update Papis Sci-Hub plugin

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Features:

• Convert plugin from Importer to Downloader
• Removed attempt to download from crossref first
• Removed dependency on scihub, instead plugin scrapes the actual webpage
  using bs4
• BibTeX metadata is pulled from https://doi.org
• Update to use pyproject.toml instead of setup.py
• Replace README.rst by README.md

Diff off original:

• Restore static versions (at least until repo starts tagging versions)
  • Add (commented-out) [tool.setuptools_scm] section to silence
    warnings once repo uses dynamic versioning
• Restore unnecessary edits
  • Restore liability warning
  • Restore version bounds (guessed for bs4)
  • Restore classifiers, keywords, authors
  • Relax python version bounds
  • Explicitly use standard GPL3 text
• Make pylint-clean for python >=3.6
  • Convert | None to uses of Optional[] to silence warnings on
    Pythons older than 3.10
  • Use more specific exceptions
  • Clean up early-exit logic, drop useless exception catches
  • Standardize names
• Misc improvements
  • Fix download_bibtex() type
  • Add sci-hub mirrors

Closes: #37

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Would've squashed the original commits to prevent some of the noise, but wanted to preserve attribution.

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Any updates on this? It should be a clean merge.