Pyflakes check is skipped on Arch Linux

Currently pyflakes.sh checks for the existence of pyflakes3 and the test is skipped if missing.

However, on Arch Linux, pyflakes3 doesn't exist because the python 3 version of pyflakes, called python-pyflakes, installs usr/bin/pyflakes and usr/bin/pyflakes3k (this is a symlink for backward compatibility, probably will be removed in the future?).

We should probably also check for pyflakes, while being careful because pyflakes on Debian/Ubuntu is the Python 2 version.