ERROR: Could not find a version that satisfies the requirement flake8 (from versions: none)
ERROR: No matching distribution found for flake8
Instead of using https://pypi.python.org, we’re using a mirror site, which is:
http://pypi.doubanio.com/simple/
The corresponding command line is

pip install package_name  -i http://pypi.doubanio.com/simple/ --trusted-host pypi.doubanio.com