[Bug]: flit publish fails with incompatible flit_core 4.x dependency

[harryswift01]

Bug Report

flit publish fails during the GitHub Actions release workflow with:

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

To Reproduce

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

- name: Publish
  run: flit publish

Build configuration:

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

Actual Output

Run flit publish
Fetching list of valid trove classifiers
Using repository at https://upload.pypi.org/legacy/

...
File ".../flit/build.py", line 46, in main
    sdist_file = sb.build(dist_dir, gen_setup_py=gen_setup_py)

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

Error: Process completed with exit code 1.

Expected Behaviour

The package should build and publish successfully to PyPI.

Environment

• GitHub Actions
• Python 3.14.6
• Flit installed with flit~=3.9