Override <number> when creating a pre-release

[BatmanAoD]

I would like to make PyO3 packages (Python packages generated from Rust code) and be able to have different prerelease suffixes from multiple branches or pull-requests at once. I thought an easy way to do this would be to include a pull-request number or other such numeric identifier in the pre-release label, but this actually causes an error when trying to generate a Python package, because -foo<num>.<num> isn't a valid Python version string according to PEP 440. So I think the only way to generate a Knope prerelease version that includes a numeric identifier like this (without resorting to --override-version and constructing the version myself) would be to have a way to just specify what number I want the pre-release label to have.