Add Python 3.11-3.13 support, drop 3.6-3.8 support

[VeckoTheGecko]

Hello! This package is one of our dependencies and I saw it could do with a bit of a refresh to bring up to speed with modern Python.
Changes:

• Add support for Python 3.11-3.13 and remove Python 3.6-3.8 (Python 3.6-3.8 have reached end of life)
  • Remove dependency on deprecated pkg_resources
    • pkg_resources is deprecated in favour of importlib.resources which was introduced in python 3.7. These changes use importlib.resources.files which was added in Python 3.9
  • Update test matrix with new Python versions
  • add init.py to rules so its seen as a module
  • Update project metadata with new versions, and link to docs

Cleanup:

• Update installation instructions (I feel that listing the dependencies isn’t that important, and can become out of sync with pyproject.toml)
• Remove .travis.yml (this looked legacy from what I could tell)
• Remove some legacy python 2.x code)
• Remove requirements.txt (this information is already covered by pyproject.toml).
• Updated contributing doc
• Update invalid escape sequence (\s)

Python 3.13 is being released next week (1 Oct) so I think let's wait until then before merging and doing a minor release!

[VeckoTheGecko]

While adding 3.12/3.13 support it would be good to also patch one (or perhaps more, I only found 1) invalid escape sequences.

e.g., \s in self.raw_text = re.sub("\n\s*(?=\n)", "", raw_text)

[VeckoTheGecko]

@castelao would you be able to run CI and review?