Stop using --use-mirrors when installing requirements.

--use-mirrors both found mirrors and accessed them insecurely, yet
trusted them (CVE-2013-5123); pip 1.5 removes support.
Installing flake8 separately is unnecessary; it is in requirements-test.txt.
Allow Genshi to be installed insecurely, for now.

Ergh. It's not like there's not been masses of warning about this.