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

[StevenMaude]

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.

[StevenMaude]

Also relates to opensafely-core/repo-template#52.