[rocky]

Update to GitHub setup and checkout @v7.
Start testing with Python 3.15

[tombouctou]

Looks good overall and all checks pass. A couple of small things:

1. macos.yml replaces 3.14 with 3.15 (['3.15', '3.12', '3.13']), so macOS no longer tests the current stable Python. Suggest ['3.15', '3.14', '3.13', '3.12'] (or at least keep 3.14 next to 3.15, as ubuntu.yml does).
2. windows.yml goes from ['3.11', '3.14'] to ['3.12', '3.15']. 3.11 is still tested on Ubuntu, so dropping it here is fine, but as with macOS nothing tests 3.14 on Windows any more.
3. Nit, from before this PR: the step in isort-and-black-checks.yml is called "Install click, black, and isort" but installs only click and black.

A one-line PR description saying why the versions were swapped would also help.

[rocky]

1. macos.yml replaces 3.14 with 3.15 (['3.15', '3.12', '3.13']), so macOS no longer tests the current stable Python. Suggest ['3.15', '3.14', '3.13', '3.12'] (or at least keep 3.14 next to 3.15, as ubuntu.yml does).

Okay done in aee726e

2. windows.yml goes from ['3.11', '3.14'] to ['3.12', '3.15']. 3.11 is still tested on Ubuntu, so dropping it here is fine, but as with macOS nothing tests 3.14 on Windows any more.

3.14 is now tested in both Windows and MacOS.

@tombouctou @mmatera, with the introduction of 3.15, should we drop testing and support for 3.11?

3. Nit, from before this PR: the step in isort-and-black-checks.yml is called "Install click, black, and isort" but installs only click and black.

click, black and isort are now installed and isort is now checked.

[tombouctou]

LGTM. All three points are addressed: macOS and Windows test 3.14 next to 3.15 again, and isort is now installed and checked (with a new .isort.cfg). All checks pass.