python requirements.txt 里指定操作系统和python版本
# ---------------------------------
# System platform value
# ---------------------------------
# AIX "aix"
# Linux "linux"
# Windows "win32"
# Windows/Cygwin "cygwin"
# MacOS "darwin"
# ---------------------------------
atomac==1.1.0; sys_platform == 'darwin'
futures>=3.0.5; python_version < '3.0'
futures>=3.0.5; python_version == '2.6' or python_version=='2.7'