Not all required dependencies specified under Windows in `requirements.dev.txt`

I just tried updating all development packages, with hashes, generated in a Linux Python 3.8 install, and found that I couldn't get the Windows build on GitHub to pass.

This is because colorama is required as a dependency for build==0.9.0 (itself a pip-tools dependency) and greater in Windows, but not for Linux.

For now, without package hashes, requirements.dev.txt still installs perfectly well in the GitHub Actions CI, because pip will quite happily install other packages it needs without hashes.

Unfortunately, pip-tools doesn't have much support for generating requirements cross-platform: jazzband/pip-tools#826. Another option is just to add colorama into the requirements, but that's not great either and also would result, I think, in this issue: pypa/build#493.

Start with requirements.dev.txt and the Windows GitHub Actions build, comparing the hashed requirements generated on Linux Python 3.8 with the dependencies needed by build==0.9.0 and pip-tools. Review pip-tools#826 and pypa/build#493 before choosing an approach; done means the Windows CI build passes while development dependencies remain correctly specified with hashes.