Bug#886456: python-pip: segfault when running 'pip install --upgrade ...'

[Nye Liu]

I think this is fixed upstream in pip 18.0

https://pip.pypa.io/en/stable/news/
"Check for file existence and unlink first when clobbering existing 
files during a wheel install. (#5366)"

Debian package is very very far behind, please update pip.