Make generating cross-plaform pins easier.

Created by: tomprince

This add colorama as a direct dependency. Currently, it a transitive dependency only on windows. When using pip-compile (from pip-tools), to pin dependencies, it only supports generating dependencies for the current platform. However, there are only two dependencies that vary between platforms, colorama and pywin32. pywin32 can't be installed on non-windows platform and has dropped python 2 support, so is relatively easy to pin. colorama is cross-platform, so including it here is a tiny bit wasteful, but makes pinning it trivial.

This is part of implementing https://whetstone.privatestorage.io/privatestorage/privatestoragedesktop/-/issues/603 for gridsync (see https://github.com/gridsync/gridsync/pull/400).