Overall package revamp

[gsakkis]

Overall package update to 2022 Python standards:

• Drop Python 2 support
• Replace pdfminer3k dependency with pdfminer.six
• Consolidate slate package to a single module
• Extend PDF from Sequence abstract class instead of list
• Add typing annotations
• Update/fix tests & doctests
  • Keep only pytest tests and move to separate dir
• Convert setup.py to project.toml
• Convert README to markdown
• Lint with black and isort
• Remove GPL LICENSE file
• Bump version to 0.6.0

[gsakkis]

@timClicks since this repo hasn't been updated since 2017, I'd be open to maintain it if you'd like to add me as collaborator.