Remove setup.py (in due course)

[hjoliver]

Ref #4507 which does this:

• Move metadata from setup.py to setup.cfg for simplicity.
• Keep the setup.py file, ...

... when we are ready we can remove this [setup.py] to switch to PEP517 build (like cylc-uiserver).

[hjoliver]

(Having seen what's left in setup.py - almost nothing - this becomes rather trivial!)

[oliver-sanders]

Removing the setup.py is trivial, however, we also have to configure the build process in the pyproject.toml in the process.

I've already done this with cylc-uiserver, as a test, it turned out to be trivial.

Suggest keeping an eye on cylc-uiserver for a bit, providing there are no hitches copy the approach over the the remaining projects.

[oliver-sanders]

Removing the setup.py file has gone without a hitch for cylc-uiserver so we may as well go for it, especially as the dual config files (.cfg & .py) seem to break Dependabot logic.

[oliver-sanders]

Tried, but hit editable-install issues and ran out of time - #5251