tox: use separate environments for pep8 and pyflakes

This should give us better errors when one but not the other fails.
Also named the environments to match the command executed.
tox: check setup.py too

When tox executes a command, it is not passed to a shell. This means
that glob patterns (e.g., *.py) wont work. We must thus explicitly
mention the Python files we want checked.
tox: specify pep8 and pyflakes deps explicitly

This avoids installing unnecessary dependencies into the virtualenvs
created for the pep8 and pyflakes tests.