Update Python version requirements and installation instructions

[Sachuriga]

Update Python version requirements and installation instructions in README and pyproject.toml

[rossant]

Thanks for working through the legacy Kwik dependencies! I think it would be simplest if phy[kwik] provided one fully working Kwik setup, including reclustering, rather than having different NumPy requirements for different parts of the GUI.

Since reclustering requires numpy<1.24, could we put numpy>=1.23,<1.24 in the kwik extra and document that the legacy Kwik GUI requires Python 3.10 or 3.11?

The main phy dependencies could then remain on numpy>=1.23 without an upper bound, and requires-python could remain >=3.10. This would keep the legacy constraints isolated to users who explicitly install phy[kwik], while allowing regular phy users to use NumPy 2 and newer Python versions.

Also, could we move the new ExampleReclusterPlugin and its associated configuration and documentation changes into a separate PR? It looks like a useful addition, but it’s independent from the Python and Kwik packaging changes. Keeping the two changes separate would make them easier to review and test.

Thanks again!