travis: test for PEP8 conformance

Include a travis specific requirements file.
PEP8: E111 indentation is not a multiple of four
PEP8: W391 blank line at end of file
PEP8: E261 at least two spaces before inline comment
PEP8: E502 the backslash is redundant between brackets
PEP8: E231 missing whitespace after ','
PEP8: E251 unexpected spaces around keyword / parameter equals
PEP8: E302 expected 2 blank lines, found 1