pep8_tap: improve formatting

Write TAP format v13, so we can add additional data to the test results,
for example the actual source!
Recommends pep8 >= 1.4.6~ as we depend on its '--format' option

pep8 backport for wheezy is as simple as:

 dget http://ftp.de.debian.org/debian/pool/main/p/pep8/pep8_1.4.6-1.1.dsc
 cd pep8-1.4.6
 dch --local ~bpo70+ --distribution wheezy-backports "Rebuild for wheezy-backports."
 debuild -us -uc -sa -S
 sudo cowbuilder --build --basepath /var/cache/pbuilder/base-wheezy-amd64.cow ../pep8_1.4.6-1.1\~bpo70+1.dsc
generate-git-snapshot: fail if git-dch doesn't support --distribution but $DIST is set