I want to open a python script from an other python scrip but in the virtualenv 
I tried `os.system` and `subprocess.call` but they couldn't open the virtualenv 

```
os.system("workon cv") warning:

sh: 1: workon: not found

cmd = 'workon cv'

subprocess.call(cmd.split())     error:

    OSError: [Errno 2] No such file or directory

```