travis: On Python3 use python3-version of pep8 #38

Previously pep8 and pyflakes were installed using the default system Python.
This changes 'make travis' to use the current Python version for those scripts as well.
This affects pep8, at least: on Python3 it complains about space in `print (...)`.
Replaced print with print_function from __future__, refs #38.
socket.py: pep8 fix on python3 #38
test__all__.py: pep8 fix on python3 #38