To be clear, in mmh3 4.* this issue should not occur, as it was introduced in #79.

The commit #79 removed MANIFEST.in, because setup.py is no longer required by setuptools. On my local environment, the default setuptools configuration in pyproject.toml included *.h files during python -m build --sdist, but on other systems, this default configuration does not include them. I’m not sure why this discrepancy occurred, but in any case this PR resolves the issue by explicitly declaring *.h as additional package data."

[tool.setuptools]
include-package-data = true
# ...

[tool.setuptools.package-data]
mmh3 = ["*.h"]