[Bug]: Update release tooling to Flit 4

[harryswift01]

Bug Report

The release configuration in this repository still uses Flit 3.x, which can resolve against an incompatible flit_core 4.x version and cause package publishing to fail.

To Reproduce

The release workflow currently uses:

- name: Install flit
  run: |
    python -m pip install --upgrade pip
    python -m pip install flit~=3.9

- name: Publish
  run: flit publish

with the build configuration:

[build-system]
requires = ["flit_core >=3.4,<4"]
build-backend = "flit_core.buildapi"

This can result in:

TypeError: SdistBuilder.build() got an unexpected keyword argument 'gen_setup_py'

Expected Behaviour

The repository should use Flit 4 consistently across the build and release configuration.

[build-system]
requires = ["flit_core >=4,<5"]
build-backend = "flit_core.buildapi"

- name: Install flit
  run: |
    python -m pip install --upgrade pip
    python -m pip install "flit>=4,<5" "flit_core>=4,<5"

- name: Publish
  run: flit publish --use-vcs

If this repository also contains a Conda recipe, its Flit Core build dependency should be updated to:

- flit-core >=4,<5