tests: use pyflakes as a tool, not a python module...
tests: use pyflakes as a tool, not a python module
The usage of pyflakes as a Python module was introduced in e397c6d74652, to work
around issue between Python 2 and Python 3. This issues are long behind us now
and we can get beck to using pyflakes as a tool, giving us more flexibility
about how we install it.
The `hghave` requirements is modified to check that we have a tool available,
instead of a python module.
