tests: use pyflakes module instead of pyflakes executable
This results in that the pyflakes version specific to the configured Python
version is used.
This way, both the Python 2-specific and the Python 3-specific pyflakes are run
by the test suite (depending on the configured Python version).
For downstream projects which are using Mercurial’s test infrastructure and are
not yet ported to Python 3 (e.g. hg-git) it ensures that the correct pyflakes
version is run even when the system’s default pyflakes is the Python 3-specific