Adopt the Python3-Pip-Skeleton

[AlexanderWells-diamond]

This PR adopts the python3-pip-skeleton . Note that this PR includes references to this project existing in the DiamondLightSource Organization. Once this PR is merged I will migrate this repo to that organization.

The (major) changes here include:

• Overhauling the GitHub actions to automatically run tests, publish new tags to PyPi, and build+release docs (note this will use GitHub Pages, not readthedocs)
• Deleting many unused files
• Adding the "tox" tool that can be used to build docs, run tests, and run some pre-commit checks
• Add some default VSCode configuration
• Change the versioning to be based on tags, using setuptools_scm. Removes the need for manual _version.py file, instead it will be autogenerated.
• Refactored docs to fit the new structure, as well as adding new ones brought over from the skeleton
• Fixed all the warnings in docs builds
• Remove explicit support for Python2 (This involved no functional changes, but I removed it from the supported list in pyproject.toml)
• Use pyproject.toml for builds, where possible. Remove all unnecessary parts from setup.py
• Move Python code to /src/cothread
• Fix an issue discovered in pvtree.py between EPICS 3 and 7

Fixes #28

[Araneidae]

@coretl , I'm afraid I cannot constructively review this; can you please give it a sanity check?

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