Download Python Packages from Requirements.txt using Pip with Trusted Host

This command downloads all the packages listed in the 'requirements.txt' file and saves them to the 'packages' directory. The '--trusted-host' option is used to indicate that the specified host (in this case, 'pypi.douban.com') is trusted, which is necessary for some network configurations. The '/usr/local/bin/pip' specifies the location of the pip executable.