In requirements.txt you can specify environment markers to conditionally select some wheel or not, for example :
pywin32 ; sys_platform == 'win32'
However, when cross-downloading like setuptools is doing, pip still uses the host platform for the environment markers instead of the platform specified on the command line (there is a change request open on pip https://github.com/pypa/pip/issues/6117)
The only workaround for setuptools looks like to have separate requirements.txt and also separate setup.py, one for each variant of dependencies ?