FIREFLY-1527: Update the package/build configuration and cleanup

[jaladh-singhal]

Fixes FIREFLY-1527, fixes #60, fixes #51

• Fixed version retrieval in code, as suggested by pkg_resources is deprecated #51
• Replaced setup.py and setup.cfg with pyproject.toml, why?
  • PyPA recommends it, see the motivation behind this change summarised at setuptools docs
  • jupyter_firefly_extensions is already using this modern config, this package should also
• Cleaned up obsolete files like travis.yml (we now have a github action to build the website) and requirements.txt (because we define dependencies in pyproject.toml)
• Updated docs pipeline to use optional [docs] dependencies
• Fixed some logical linting errors in source files
• Updated release documentation as per build config changes

Testing

Checkout this branch locally and then in a fresh local venv or conda env, doing pip install . should install firefly_client. Then run example notebooks - they should work

[bsipocz]

The notebooks now pass all the tests with OSX, as well as without installing setuptools, so from my end this looks good.

[robyww]

firefly_client.py and env.py clean up looks good. Documentation updates look good.

I am glad we are staying up-to-date on packaging.