fix virtualenv support

virtualenv site-packages is inserted every time when calling `jedi.evaluate.sys_path.get_sys_path()`, and it is called by Jedi many times on complete.
After some completions, sys.path is extended by too many virtualenv site-packages'.

On Windows, virtualenv site-packages is located under lib directory
ref: https://github.com/pypa/virtualenv/blob/1.11.5/virtualenv.py#L1225