gunicorn Error: class uri 'gunicorn.workers.gthread.ThreadWorker' invalid or not found

今天在一台新的机子上运行guicorn的时候报了一个错

Error: class uri 'gunicorn.workers.gthread.ThreadWorker' invalid or not found:

[Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/gunicorn/util.py", line 135, in load_class
    mod = import_module('.'.join(components))
  File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/local/lib/python2.7/dist-packages/gunicorn/workers/gthread.py", line 37, in 
    """)
RuntimeError:
    You need to install the 'futures' package to use this worker with this
    Python version.

]

上面显示说需要安装futures包，于是

pip install futures
pip3 install futures

然而都显示说这个包已经安装了
google了一下，gunicorn的原作者两年前说这个问题已经修复了，但是我看了一下，我的gunicorn是最新版的没错，又看了一下报错信息，显示错误来自python2.7里面的gunicorn包，于是我又试了一下

pip uninstall gunicorn

pip3 install guncorn

再运行，报错消失