Make pep8 tests run inside virtualenv

Running pep8 results in the following error:

Traceback (most recent call last):
File "tools/hacking.py", line 34, in <module>
import pep8
ImportError: No module named pep8

Change-Id: I31a8bfbffa9ddee306ab414fab367c74d34fdd6b
Merge "Make pep8 tests run inside virtualenv"