Using Tox on TravisCI to enforce pep8 standards etc

Hello all,

Many of us are already using the git pre-commit hook
setup by Christian Brueffer, but new contributors in
particular are unlikely to have this installed - and I
still don't know how to use this on Windows:

https://github.com/biopython/biopython/issues/493

I think it would be sensible to include the pep8 checks
as part of our Travis Continuous Integration checks,
which would then automatically be applied to pull
requests as well.

Ideally I would like the pre-commit hook to be strict but
optional (you can override this when committing), while
the TravisCI/Tox configuration should be more lenient
but strictly enforced:

https://github.com/biopython/biopython/issues/840

The PEP8 standards are guidelines, and as you will
see from the proposed TravisCI/Tox configuration and
the existing pre-commit hook, there are many that we
do not follow.

There is more than one way to add this to TravisCI.
Having looked at this I think using Tox under TravisCI
is a workable plan:

https://github.com/biopython/biopython/pull/877

This pull request restructures our existing TravisCI
configuration to install and use Tox. The motivation
is to add extra test targets which would have be
more complicated otherwise.

The main new target is to run the pep8 tool to enforce
coding standards. Two other new TravisCI test targets
are to run the "python setup.py sdist" commands we
use when building a release, and also check
"python setup.py bdist_wheel" to build a binary wheel
which I would like to include in future release processes.

What do people think?

We already try to point out style issues when doing
pull request reviews, so having this partly automated
via pep8 on TravisCI should help.

Comments please,

Peter