Retry pip install a few times

There's a race condition in Cython's setup script that makes
'pip install Cython' fail once in a blue moon: 
https://groups.google.com/forum/#!msg/cython-users/bKYut7C9t7k/15MKCquPZLEJ

Retrying should make it succeed and avoid false failures on pull requests.

(I also dropped the deprecated --use-mirrors option of pip install.)