Combined requirements.txt with automatic OS install selection

[cooperdk]

Instead of your current three-file setup, do only one requirements.txt with the below contents.

When doing pip install -r requirements.txt --test, the packages with the option in the requirements.txt file will be applied, otherwise not. And, there is a selector for specific operating systems. This way you can combine it all in one file.

Send2Trash>=1.3.0
sphinx>=1.2.2
polib>=1.0.4
hsaudiotag3k>=1.1.3
distro>=1.5.0
pytest>=5,<6 --install-option='--test'
flake8 --install-option='--test'
tox-travis --install-option='--test'
black --install-option='--test'
PyQt5 >=5.4,<6.0; platform_system == "Windows"
pywin32>=200; platform_system == "Windows"
pyinstaller>=3.4,<4.0; platform_system == "Windows"

[arsenetar]

Looks like doing this would force pip to not use wheels due to issue https://github.com/pypa/pip/issues/4118, which seems like it ends up being a regression over what we have now. Otherwise I would have been in favor of this.